I've been having more luck with the newer 4.7 and 4.8 kernels, and using a 4k monitor on the integrated DP connector mostly works okay at 60Hz.

Haven't really noticed any difference in stability with the xorg.conf.d/20-intel.conf settings in https://github.com/linuxenko/ubuntu-skylake-i915-video-fix. I also tried with `Option "DRI" "false"`.

Currently running 4.8.0-040800rc5-generic together with yakkety linux-firmware 1.160:

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                              Version               Architecture          Description
+++-=================================-=====================-=====================-========================================================================
ii  linux-firmware                    1.160                 all                   Firmware for Linux kernel drivers
ii  linux-image-4.8.0-040800rc5-gener 4.8.0-040800rc5.20160 amd64                 Linux kernel image for version 4.8.0 on 64 bit x86 SMP

Still getting warnings in dmesg when connecting a DP monitor:

[         ...] Linux version 4.8.0-040800rc5-generic (kernel@tangerine) (gcc version 6.2.0 20160830 (Ubuntu 6.2.0-2ubuntu11) ) #201609041832 SMP Sun Sep 4 22:34:01 UTC 2016
....
[  330.453739] ------------[ cut here ]------------
[  330.453793] WARNING: CPU: 0 PID: 5134 at /home/kernel/COD/linux/drivers/gpu/drm/drm_irq.c:1215 drm_wait_one_vblank+0x16b/0x1b0 [drm]
[  330.453796] vblank not available on crtc 0, ret=-22
[  330.453798] Modules linked in: uas usb_storage hid_lenovo usbhid hid ctr ccm pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) rfcomm fuse ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 xt_addrtype iptable_filter ip_tables xt_conntrack x_tables nf_nat nf_conntrack br_netfilter bridge stp llc bnep snd_hda_codec_hdmi ext4 jbd2 arc4 snd_hda_codec_realtek snd_hda_codec_generic iwlmvm mac80211 fscrypto intel_rapl mbcache x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm irqbypass intel_cstate intel_rapl_perf joydev serio_raw iwlwifi cfg80211 rtsx_pci_ms memstick option usb_wwan usbserial btusb btrtl cdc_ether btbcm usbnet btintel mii bluetooth sg crc16 snd_soc_skl snd_soc_skl_ipc snd_soc_sst_ipc snd_soc_sst_dsp snd_hda_ext_core snd_soc_sst_match
[  330.453921]  snd_soc_core snd_compress snd_pcm_dmaengine snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd_pcm thinkpad_acpi nvram rfkill battery ac snd_seq_midi snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer snd evdev tpm_tis tpm_tis_core tpm soundcore mei_me mei shpchp intel_pch_thermal parport_pc ppdev lp parport autofs4 btrfs xor raid6_pq algif_skcipher af_alg dm_crypt dm_mod sd_mod rtsx_pci_sdmmc mmc_core crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd i915 psmouse xhci_pci rtsx_pci xhci_hcd mfd_core ahci i2c_algo_bit usbcore libahci drm_kms_helper e1000e syscopyarea libata usb_common sysfillrect sysimgblt fb_sys_fops ptp pps_core drm scsi_mod thermal wmi video fjes button
[  330.454049] CPU: 0 PID: 5134 Comm: Xorg Tainted: G        W  OE   4.8.0-040800rc5-generic #201609041832
[  330.454052] Hardware name: LENOVO 20F6007RGE/20F6007RGE, BIOS R02ET48W (1.21 ) 06/01/2016
[  330.454057]  0000000000000286 000000000d9272e2 ffffffffb2541064 ffff9487a2e239c8
[  330.454068]  0000000000000000 ffffffffb227f6ee ffff9487c7f30000 ffff9487a2e23a20
[  330.454076]  ffff9487c85a1600 ffff9487c7f303d8 ffff9487c7f30000 0000000000000000
[  330.454085] Call Trace:
[  330.454099]  [<ffffffffb2541064>] ? dump_stack+0x5c/0x78
[  330.454108]  [<ffffffffb227f6ee>] ? __warn+0xbe/0xe0
[  330.454117]  [<ffffffffb227f76f>] ? warn_slowpath_fmt+0x5f/0x80
[  330.454155]  [<ffffffffc01b7156>] ? drm_vblank_get+0x76/0xc0 [drm]
[  330.454192]  [<ffffffffc01b739b>] ? drm_wait_one_vblank+0x16b/0x1b0 [drm]
[  330.454286]  [<ffffffffc04a1890>] ? chv_write32+0x3c0/0x3c0 [i915]
[  330.454344]  [<ffffffffc0451a8d>] ? skl_wm_flush_pipe+0xcd/0x100 [i915]
[  330.454400]  [<ffffffffc04527cb>] ? skl_update_wm+0x42b/0x6c0 [i915]
[  330.454493]  [<ffffffffc04c46a8>] ? haswell_crtc_enable+0x798/0x860 [i915]
[  330.454584]  [<ffffffffc04c071f>] ? intel_atomic_commit_tail+0x84f/0x10a0 [i915]
[  330.454671]  [<ffffffffc04c9040>] ? intel_prepare_plane_fb+0x100/0x2b0 [i915]
[  330.454698]  [<ffffffffc0313a42>] ? drm_atomic_helper_setup_commit+0x252/0x320 [drm_kms_helper]
[  330.454784]  [<ffffffffc04c13b2>] ? intel_atomic_commit+0x442/0x560 [i915]
[  330.454843]  [<ffffffffc01cff22>] ? drm_atomic_set_crtc_for_connector+0x92/0xf0 [drm]
[  330.454870]  [<ffffffffc0314789>] ? drm_atomic_helper_set_config+0x79/0xb0 [drm_kms_helper]
[  330.454919]  [<ffffffffc01be671>] ? drm_mode_set_config_internal+0x61/0x110 [drm]
[  330.454969]  [<ffffffffc01c312b>] ? drm_mode_setcrtc+0x42b/0x560 [drm]
[  330.455004]  [<ffffffffc01b5b3b>] ? drm_ioctl+0x2ab/0x460 [drm]
[  330.455051]  [<ffffffffc01c2d00>] ? drm_mode_setplane+0x1c0/0x1c0 [drm]
[  330.455059]  [<ffffffffb2424faf>] ? do_vfs_ioctl+0x9f/0x640
[  330.455069]  [<ffffffffb228c1f7>] ? recalc_sigpending+0x17/0x50
[  330.455078]  [<ffffffffb228cf8d>] ? __set_task_blocked+0x3d/0x90
[  330.455083]  [<ffffffffb24255c4>] ? SyS_ioctl+0x74/0x80
[  330.455094]  [<ffffffffb281eb76>] ? entry_SYSCALL_64_fastpath+0x1e/0xa8
[  330.455099] ---[ end trace f3839ca74b0e45b3 ]---
[  591.794245] [drm:gen8_irq_handler [i915]] *ERROR* CPU pipe A FIFO underrun

However, using the DP connector on a ThinkPad Pro Dock fails miserably with the mainline 4.8-rc kernels. I did get the display working via the dock connector the first time, but not anymore with the latest 4.8-rc kernels :(

dmesg after a suspend/resume cycle:

thinkpad_acpi: docked into hotplug port replicator
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
usb 2-4: new SuperSpeed USB device number 12 using xhci_hcd
usb 2-4: New USB device found, idVendor=17ef, idProduct=1012
usb 2-4: New USB device strings: Mfr=1, Product=2, SerialNumber=0
usb 2-4: Product: Lenovo ThinkPad Dock   
usb 2-4: Manufacturer: LENOVO                 
hub 2-4:1.0: USB hub found
hub 2-4:1.0: 4 ports detected
usb 1-4: new high-speed USB device number 72 using xhci_hcd
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
usb 1-4: New USB device found, idVendor=17ef, idProduct=1012
usb 1-4: New USB device strings: Mfr=1, Product=2, SerialNumber=0
usb 1-4: Product: Lenovo ThinkPad Dock   
usb 1-4: Manufacturer: LENOVO                 
hub 1-4:1.0: USB hub found
hub 1-4:1.0: 4 ports detected
thinkpad_acpi: EC reports that Thermal Table has changed
usb 1-4.4: new high-speed USB device number 73 using xhci_hcd
<warn>  (ttyUSB4): port attributes not fully set
<warn>  (ttyUSB2): port attributes not fully set
<warn>  (ttyUSB3): port attributes not fully set
<warn>  (ttyUSB1): port attributes not fully set
usb 1-4.4: New USB device found, idVendor=17ef, idProduct=1011
usb 1-4.4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 1-4.4: Product: Lenovo ThinkPad Dock
usb 1-4.4: Manufacturer: Lenovo
usb 1-4.4: SerialNumber: Rev1.2
hub 1-4.4:1.0: USB hub found
hub 1-4.4:1.0: 3 ports detected
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_dp_start_link_train [i915]] *ERROR* failed to train DP, aborting
[drm:intel_dp_start_link_train [i915]] *ERROR* Timed out waiting for DP idle patterns
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_ddi_prepare_link_retrain [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage retries, give up
[drm:intel_dp_start_link_train [i915]] *ERROR* failed to train DP, aborting
[drm:intel_dp_start_link_train [i915]] *ERROR* Timed out waiting for DP idle patterns

This matches with: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1568573

dmesg after initial boot:

syys 05 10:12:57 tehobari kernel: thinkpad_acpi: docked into hotplug port replicator
syys 05 10:12:59 tehobari kernel: ------------[ cut here ]------------
syys 05 10:12:59 tehobari kernel: WARNING: CPU: 1 PID: 5134 at /home/kernel/COD/linux/drivers/gpu/drm/drm_irq.c:1215 drm_wait_one_vblank+0x16b/0x1b0 [drm]
syys 05 10:12:59 tehobari kernel: vblank not available on crtc 0, ret=-22
syys 05 10:12:59 tehobari kernel: Modules linked in: ctr ccm pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) rfcomm fuse ipt_MASQUERADE nf_nat_
syys 05 10:12:59 tehobari kernel:  snd_pcm_dmaengine snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd_pcm thinkpad_acpi nvram rfkill battery ac snd_seq_
syys 05 10:12:59 tehobari kernel: CPU: 1 PID: 5134 Comm: Xorg Tainted: G        W  OE   4.8.0-040800rc5-generic #201609041832
syys 05 10:12:59 tehobari kernel: Hardware name: LENOVO 20F6007RGE/20F6007RGE, BIOS R02ET48W (1.21 ) 06/01/2016
syys 05 10:12:59 tehobari kernel:  0000000000000286 000000000d9272e2 ffffffffb2541064 ffff9487a2e239c8
syys 05 10:12:59 tehobari kernel:  0000000000000000 ffffffffb227f6ee ffff9487c7f30000 ffff9487a2e23a20
syys 05 10:12:59 tehobari kernel:  ffff9487c85a1600 ffff9487c7f303d8 ffff9487c7f30000 0000000000000000
syys 05 10:12:59 tehobari kernel: Call Trace:
syys 05 10:12:59 tehobari kernel:  [<ffffffffb2541064>] ? dump_stack+0x5c/0x78
syys 05 10:12:59 tehobari kernel:  [<ffffffffb227f6ee>] ? __warn+0xbe/0xe0
syys 05 10:12:59 tehobari kernel:  [<ffffffffb227f76f>] ? warn_slowpath_fmt+0x5f/0x80
syys 05 10:12:59 tehobari kernel:  [<ffffffffc01b7156>] ? drm_vblank_get+0x76/0xc0 [drm]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc01b739b>] ? drm_wait_one_vblank+0x16b/0x1b0 [drm]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc04a1890>] ? chv_write32+0x3c0/0x3c0 [i915]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc0451a8d>] ? skl_wm_flush_pipe+0xcd/0x100 [i915]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc04527cb>] ? skl_update_wm+0x42b/0x6c0 [i915]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc04c46a8>] ? haswell_crtc_enable+0x798/0x860 [i915]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc04c071f>] ? intel_atomic_commit_tail+0x84f/0x10a0 [i915]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc04c9040>] ? intel_prepare_plane_fb+0x100/0x2b0 [i915]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc0313a42>] ? drm_atomic_helper_setup_commit+0x252/0x320 [drm_kms_helper]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc04c13b2>] ? intel_atomic_commit+0x442/0x560 [i915]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc01cff22>] ? drm_atomic_set_crtc_for_connector+0x92/0xf0 [drm]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc0314789>] ? drm_atomic_helper_set_config+0x79/0xb0 [drm_kms_helper]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc01be671>] ? drm_mode_set_config_internal+0x61/0x110 [drm]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc01c312b>] ? drm_mode_setcrtc+0x42b/0x560 [drm]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc01b5b3b>] ? drm_ioctl+0x2ab/0x460 [drm]
syys 05 10:12:59 tehobari kernel:  [<ffffffffc01c2d00>] ? drm_mode_setplane+0x1c0/0x1c0 [drm]
syys 05 10:12:59 tehobari kernel:  [<ffffffffb2424faf>] ? do_vfs_ioctl+0x9f/0x640
syys 05 10:12:59 tehobari kernel:  [<ffffffffb228c1f7>] ? recalc_sigpending+0x17/0x50
syys 05 10:12:59 tehobari kernel:  [<ffffffffb228cf8d>] ? __set_task_blocked+0x3d/0x90
syys 05 10:12:59 tehobari kernel:  [<ffffffffb24255c4>] ? SyS_ioctl+0x74/0x80
syys 05 10:12:59 tehobari kernel:  [<ffffffffb281eb76>] ? entry_SYSCALL_64_fastpath+0x1e/0xa8
syys 05 10:12:59 tehobari kernel: ---[ end trace f3839ca74b0e45b1 ]---
syys 05 10:12:59 tehobari kernel: thinkpad_acpi: EC reports that Thermal Table has changed
syys 05 10:12:59 tehobari kernel: [drm:intel_mst_pre_enable_dp [i915]] *ERROR* failed to allocate vcpi
syys 05 10:12:59 tehobari kernel: [drm:intel_mst_enable_dp [i915]] *ERROR* Timed out waiting for ACT sent

The reference to MST here makes me think this is probably a different bug..