Hello, I also experienced it a few times for last few days. Interesting facts:
- I am running archlinux (so it's not ubuntu specific)
- It is running on a Zotac hardware (working as a HTPC)
- it was running XBMC playing a movie when freezed
- it was running kernel 3.17.4

[41772.634908] ------------[ cut here ]------------
[41772.634969] WARNING: CPU: 0 PID: 483 at drivers/gpu/drm/i915/intel_display.c:3361 intel_crtc_wait_for_pending_flips+0x1a1/0x1b0 [i915]()
[41772.634973] Modules linked in: nls_utf8 udf crc_itu_t bnep arc4 snd_hda_codec_hdmi uvcvideo rt2800pci rt2800mmio rt2800lib rt2x00pci snd_hda_codec_realtek videobuf2_vmalloc snd_usb_audio videobuf2_memops rt2x00mmio rt2x00lib videobuf2_core snd_usbmidi_lib snd_hda_codec_generic led_class v4l2_common ecb snd_rawmidi videodev mac80211 btusb snd_seq_device cfg80211 snd_hda_intel snd_hda_controller bluetooth snd_hda_codec ums_realtek media eeprom_93cx6 crc_ccitt rfkill snd_hwdep joydev snd_pcm snd_timer snd coretemp hwmon intel_rapl i915 iTCO_wdt x86_pkg_temp_thermal r8169 intel_powerclamp evdev iTCO_vendor_support soundcore kvm_intel mii mac_hid serio_raw psmouse pcspkr drm_kms_helper kvm crct10dif_pclmul drm crc32_pclmul crc32c_intel ghash_clmulni_intel intel_gtt cryptd mei_me battery thermal video
[41772.635142]  button shpchp i2c_algo_bit i2c_i801 i2c_core mei lpc_ich processor fan sch_fq_codel ext4 crc16 mbcache jbd2 sr_mod cdrom hid_generic usbhid hid uas usb_storage sd_mod crc_t10dif crct10dif_common atkbd libps2 ahci libahci xhci_hcd ehci_pci ehci_hcd libata scsi_mod usbcore usb_common i8042 serio
[41772.635205] CPU: 0 PID: 483 Comm: Xorg.bin Not tainted 3.17.4-1-ARCH #1
[41772.635209] Hardware name: ZOTAC XXXXXX/XXXXXX, BIOS 4.6.4 06/28/2012
[41772.635214]  0000000000000000 000000002f4774f5 ffff8800d4c23b98 ffffffff81537b0e
[41772.635221]  0000000000000000 ffff8800d4c23bd0 ffffffff8107078d 0000000000000000
[41772.635228]  ffff8800d5b78288 ffff880118da8000 ffff8800d580e800 0000000000000000
[41772.635235] Call Trace:
[41772.635249]  [<ffffffff81537b0e>] dump_stack+0x4d/0x6f
[41772.635259]  [<ffffffff8107078d>] warn_slowpath_common+0x7d/0xa0
[41772.635267]  [<ffffffff810708ba>] warn_slowpath_null+0x1a/0x20
[41772.635297]  [<ffffffffa049b3f1>] intel_crtc_wait_for_pending_flips+0x1a1/0x1b0 [i915]
[41772.635308]  [<ffffffff810af950>] ? __wake_up_sync+0x20/0x20
[41772.635335]  [<ffffffffa04a37d4>] intel_crtc_disable_planes+0x34/0x160 [i915]
[41772.635360]  [<ffffffffa04a3f30>] ironlake_crtc_disable+0x50/0x7b0 [i915]
[41772.635369]  [<ffffffff81151685>] ? filemap_map_pages+0x135/0x250
[41772.635394]  [<ffffffffa049ccb9>] intel_crtc_control+0x49/0x100 [i915]
[41772.635418]  [<ffffffffa049cdd7>] intel_crtc_update_dpms+0x67/0x80 [i915]
[41772.635444]  [<ffffffffa04a5ed9>] intel_connector_dpms+0x59/0x70 [i915]
[41772.635465]  [<ffffffffa02ef6c9>] drm_mode_obj_set_property_ioctl+0x399/0x3a0 [drm]
[41772.635483]  [<ffffffffa02ef70f>] drm_mode_connector_property_set_ioctl+0x3f/0x60 [drm]
[41772.635501]  [<ffffffffa02dffef>] drm_ioctl+0x1df/0x680 [drm]
[41772.635512]  [<ffffffff8105e9ac>] ? __do_page_fault+0x2ec/0x600
[41772.635521]  [<ffffffff8101e439>] ? read_tsc+0x9/0x10
[41772.635529]  [<ffffffff811da5f0>] do_vfs_ioctl+0x2d0/0x4b0
[41772.635536]  [<ffffffff811e4cae>] ? __fget+0x6e/0xb0
[41772.635544]  [<ffffffff811da851>] SyS_ioctl+0x81/0xa0
[41772.635553]  [<ffffffff8153db29>] system_call_fastpath+0x16/0x1b
[41772.635557] ---[ end trace 39f745eed7a501b7 ]---
[root@asrock ~]#