I noticed this syslog output just before the crash:
Oct 28 21:31:35 bologna lightdm[1388]: X Error of failed request:  BadValue (integer parameter out of range for operation)
Oct 28 21:31:35 bologna lightdm[1388]: Major opcode of failed request:  140 (RANDR)
Oct 28 21:31:35 bologna lightdm[1388]: Minor opcode of failed request:  35 (RRSetProviderOutputSource)
Oct 28 21:31:35 bologna lightdm[1388]: Value in failed request:  0x46
Oct 28 21:31:35 bologna lightdm[1388]: Serial number of failed request:  15
Oct 28 21:31:35 bologna lightdm[1388]: Current serial number in output stream:  16
Oct 28 21:31:36 bologna kernel: [ 6870.862956] [drm:intel_pipe_config_compare [i915]] *ERROR* mismatch in ips_enabled (expected 1, found 0)
Oct 28 21:31:36 bologna kernel: [ 6870.862960] ------------[ cut here ]------------
Oct 28 21:31:36 bologna kernel: [ 6870.862975] WARNING: CPU: 3 PID: 7966 at /build/linux-AxjFAn/linux-4.2.0/drivers/gpu/drm/i915/intel_display.c:12326 check_crtc_state+0x2c5/0x440 [i915]()
Oct 28 21:31:36 bologna kernel: [ 6870.862977] pipe state doesn't match!
Oct 28 21:31:36 bologna kernel: [ 6870.862978] Modules linked in: rfcomm drbg ansi_cprng ctr ccm bnep pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) nls_iso8859_1 nvidia(POE) uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_core v4l2_common arc4 videodev media iwlmvm mac80211 btusb btr
tl btbcm btintel bluetooth hid_multitouch dell_wmi sparse_keymap iwlwifi dell_laptop dcdbas dell_smm_hwmon snd_hda_codec_hdmi intel_rapl iosf_mbi x86_pkg_temp_thermal intel_powerclamp coretemp dell_led rtsx_pci_ms kvm_intel kvm snd_hda_codec_realtek snd_hda_codec_generic snd_soc_rt5640 snd_soc_rl6231 snd_s
oc_core snd_hda_intel crct10dif_pclmul snd_hda_codec crc32_pclmul snd_compress ghash_clmulni_intel ac97_bus snd_hda_core snd_pcm_dmaengine snd_hwdep aesni_intel aes_x86_64 snd_pcm lrw gf128mul joydev glue_helper input_leds ablk_helper snd_seq_midi cryptd snd_seq_midi_event memstick serio_raw snd_rawmidi sn
d_seq cfg80211 snd_seq_device snd_timer snd soundcore wmi i2c_designware_platform i2c_designware_core spi_pxa2xx_platform intel_smartconnect dell_smo8800 dell_rbtn 8250_dw snd_soc_sst_acpi dw_dmac dw_dmac_core mei_me shpchp mei mac_hid lpc_ich sunrpc parport_pc ppdev lp parport autofs4 hid_logitech_hidpp h
id_logitech_dj hid_generic usbhid uas usb_storage rtsx_pci_sdmmc i915 i2c_algo_bit psmouse drm_kms_helper ahci drm r8169 libahci rtsx_pci mii video sdhci_acpi i2c_hid sdhci hid
Oct 28 21:31:36 bologna kernel: [ 6870.863044] CPU: 3 PID: 7966 Comm: Xorg Tainted: P        W  OE   4.2.0-16-generic #19-Ubuntu
Oct 28 21:31:36 bologna kernel: [ 6870.863045] Hardware name: Dell Inc.          Inspiron 7537/038M0M, BIOS A12 10/30/2014
Oct 28 21:31:36 bologna kernel: [ 6870.863047]  0000000000000000 00000000e0111a78 ffff8803e5e6f7c8 ffffffff817e8c09
Oct 28 21:31:36 bologna kernel: [ 6870.863049]  0000000000000000 ffff8803e5e6f820 ffff8803e5e6f808 ffffffff8107b3c6
Oct 28 21:31:36 bologna kernel: [ 6870.863051]  ffff8803e5e6f848 ffff8803e5e6f8b0 ffff880427bc4800 0000000000000001
Oct 28 21:31:36 bologna kernel: [ 6870.863054] Call Trace:
Oct 28 21:31:36 bologna kernel: [ 6870.863059]  [<ffffffff817e8c09>] dump_stack+0x45/0x57
Oct 28 21:31:36 bologna kernel: [ 6870.863063]  [<ffffffff8107b3c6>] warn_slowpath_common+0x86/0xc0
Oct 28 21:31:36 bologna kernel: [ 6870.863065]  [<ffffffff8107b455>] warn_slowpath_fmt+0x55/0x70
Oct 28 21:31:36 bologna kernel: [ 6870.863079]  [<ffffffffc01efbc1>] ? intel_pipe_config_compare+0xb31/0xc60 [i915]
Oct 28 21:31:36 bologna kernel: [ 6870.863091]  [<ffffffffc01effb5>] check_crtc_state+0x2c5/0x440 [i915]
Oct 28 21:31:36 bologna kernel: [ 6870.863106]  [<ffffffffc0203d6d>] intel_modeset_check_state+0x21d/0x6d0 [i915]
Oct 28 21:31:36 bologna kernel: [ 6870.863119]  [<ffffffffc0204f27>] intel_crtc_set_config+0x4c7/0x580 [i915]
Oct 28 21:31:36 bologna kernel: [ 6870.863131]  [<ffffffffc00bbee5>] ? drm_mode_create+0x25/0x60 [drm]
Oct 28 21:31:36 bologna kernel: [ 6870.863140]  [<ffffffffc00b54c6>] drm_mode_set_config_internal+0x66/0x100 [drm]
Oct 28 21:31:36 bologna kernel: [ 6870.863150]  [<ffffffffc00b9b29>] drm_mode_setcrtc+0x3e9/0x500 [drm]
Oct 28 21:31:36 bologna kernel: [ 6870.863156]  [<ffffffffc00aa495>] drm_ioctl+0x125/0x610 [drm]
Oct 28 21:31:36 bologna kernel: [ 6870.863159]  [<ffffffff8123e696>] ? fsnotify+0x316/0x4a0
Oct 28 21:31:36 bologna kernel: [ 6870.863167]  [<ffffffffc00b9740>] ? drm_mode_setplane+0x1b0/0x1b0 [drm]
Oct 28 21:31:36 bologna kernel: [ 6870.863171]  [<ffffffff812108a5>] do_vfs_ioctl+0x295/0x480
Oct 28 21:31:36 bologna kernel: [ 6870.863174]  [<ffffffff81087f61>] ? __set_task_blocked+0x41/0xa0
Oct 28 21:31:36 bologna kernel: [ 6870.863176]  [<ffffffff81210b09>] SyS_ioctl+0x79/0x90
Oct 28 21:31:36 bologna kernel: [ 6870.863179]  [<ffffffff8108ab8e>] ? SyS_rt_sigprocmask+0x8e/0xc0
Oct 28 21:31:36 bologna kernel: [ 6870.863183]  [<ffffffff817ef9f2>] entry_SYSCALL_64_fastpath+0x16/0x75
Oct 28 21:31:36 bologna kernel: [ 6870.863184] ---[ end trace 4d14a9175e944d86 ]---

[more stacktraces of other CPUs]