Here the relevant messages from syslog:

Dec 19 14:25:12 localhost kernel: [ 4965.409518] BUG: unable to handle kernel NULL pointer dereference at 00000048
Dec 19 14:25:12 localhost kernel: [ 4965.409572] IP: [<f810edd1>] drm_mm_scan_add_block+0x11/0x100 [drm]
Dec 19 14:25:12 localhost kernel: [ 4965.409622] *pde = 7c79e067 
Dec 19 14:25:12 localhost kernel: [ 4965.409639] Oops: 0002 [#1] SMP 
Dec 19 14:25:12 localhost kernel: [ 4965.409663] Modules linked in: rfcomm bnep bluetooth dm_crypt binfmt_misc snd_hda_codec_realtek snd_hda_intel snd_hda_codec snd_hwdep snd_pcm snd_seq_midi snd_rawmidi usbhid hid snd_seq_midi_event snd_seq snd_timer snd_seq_device snd soundcore ppdev snd_page_alloc psmouse serio_raw parport_pc w83627ehf hwmon_vid coretemp lp parport i915 floppy r8169 drm_kms_helper drm i2c_algo_bit video
Dec 19 14:25:12 localhost kernel: [ 4965.409887] 
Dec 19 14:25:12 localhost kernel: [ 4965.409897] Pid: 4489, comm: Xorg Not tainted 3.2.0-5-generic #11-Ubuntu FUJITSU SIEMENS ESPRIMO P3510                 /D2750-A2
Dec 19 14:25:12 localhost kernel: [ 4965.409960] EIP: 0060:[<f810edd1>] EFLAGS: 00013286 CPU: 1
Dec 19 14:25:12 localhost kernel: [ 4965.409996] EIP is at drm_mm_scan_add_block+0x11/0x100 [drm]
Dec 19 14:25:12 localhost kernel: [ 4965.410026] EAX: f68c2e0c EBX: f3cbf1bc ECX: 00000000 EDX: efe4dd40
Dec 19 14:25:12 localhost kernel: [ 4965.410058] ESI: f3cbf1bc EDI: f3cbf22c EBP: efe4dcf4 ESP: efe4dcd8
Dec 19 14:25:12 localhost kernel: [ 4965.410089]  DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
Dec 19 14:25:12 localhost kernel: [ 4965.410117] Process Xorg (pid: 4489, ti=efe4c000 task=efe99960 task.ti=efe4c000)
Dec 19 14:25:12 localhost kernel: [ 4965.410156] Stack:
Dec 19 14:25:12 localhost kernel: [ 4965.410167]  00000800 00800000 00800000 04300000 f3cbf1bc f3cbf1bc f3cbf22c efe4dd00
Dec 19 14:25:12 localhost kernel: [ 4965.410216]  f829a90a efe4dd40 efe4dd54 f829aa0f 00000000 10000000 00000000 00000480
Dec 19 14:25:12 localhost kernel: [ 4965.410265]  00000180 00000000 00000001 00000c0e f3cbf190 01207990 00800000 f3cbe000
Dec 19 14:25:12 localhost kernel: [ 4965.410314] Call Trace:
Dec 19 14:25:12 localhost kernel: [ 4965.410340]  [<f829a90a>] mark_free+0x2a/0x40 [i915]
Dec 19 14:25:12 localhost kernel: [ 4965.410375]  [<f829aa0f>] i915_gem_evict_something+0xef/0x370 [i915]
Dec 19 14:25:12 localhost kernel: [ 4965.410417]  [<f82965f4>] i915_gem_object_bind_to_gtt+0x1b4/0x400 [i915]
Dec 19 14:25:12 localhost kernel: [ 4965.410462]  [<f829918c>] i915_gem_object_pin+0x13c/0x1a0 [i915]
Dec 19 14:25:12 localhost kernel: [ 4965.410498]  [<c105a0a5>] ? mod_timer+0x1e5/0x280
Dec 19 14:25:12 localhost kernel: [ 4965.410534]  [<f82be351>] ? ring_write_tail+0x21/0x30 [i915]
Dec 19 14:25:12 localhost kernel: [ 4965.410573]  [<f829b6e2>] i915_gem_execbuffer_reserve.isra.5+0x262/0x340 [i915]
Dec 19 14:25:12 localhost kernel: [ 4965.410619]  [<f829bf96>] i915_gem_do_execbuffer.isra.7+0x496/0x7c0 [i915]
Dec 19 14:25:12 localhost kernel: [ 4965.410658]  [<c102bf95>] ? iounmap_atomic+0x75/0xa0
Dec 19 14:25:12 localhost kernel: [ 4965.410687]  [<c1122c78>] ? __kmalloc+0x1d8/0x1e0
Dec 19 14:25:12 localhost kernel: [ 4965.410720]  [<f829c7dd>] i915_gem_execbuffer2+0x5d/0x210 [i915]
Dec 19 14:25:12 localhost kernel: [ 4965.410754]  [<c12a2e1d>] ? __copy_from_user_ll+0x2d/0x40
Dec 19 14:25:12 localhost kernel: [ 4965.410788]  [<f8104e18>] drm_ioctl+0x378/0x480 [drm]
Dec 19 14:25:12 localhost kernel: [ 4965.410824]  [<f829c780>] ? i915_gem_execbuffer+0x4c0/0x4c0 [i915]
Dec 19 14:25:12 localhost kernel: [ 4965.410860]  [<c1572e70>] ? vmalloc_fault+0xee/0xee
Dec 19 14:25:12 localhost kernel: [ 4965.410886]  [<c1573097>] ? do_page_fault+0x227/0x490
Dec 19 14:25:12 localhost kernel: [ 4965.410918]  [<f8104aa0>] ? drm_copy_field+0x80/0x80 [drm]
Dec 19 14:25:12 localhost kernel: [ 4965.410949]  [<c1142f29>] do_vfs_ioctl+0x79/0x2d0
Dec 19 14:25:12 localhost kernel: [ 4965.410974]  [<c11431ef>] sys_ioctl+0x6f/0x80
Dec 19 14:25:12 localhost kernel: [ 4965.410999]  [<c156fcb4>] syscall_call+0x7/0xb
Dec 19 14:25:12 localhost kernel: [ 4965.411022] Code: c8 89 45 f0 89 44 24 10 e8 b2 bc 44 c9 8b 55 e8 03 5d f0 eb c2 90 8d 74 26 00 55 89 e5 57 56 53 83 ec 10 3e 8d 74 26 00 8b 48 1c 
Dec 19 14:25:12 localhost kernel: [ 4965.411203] EIP: [<f810edd1>] drm_mm_scan_add_block+0x11/0x100 [drm] SS:ESP 0068:efe4dcd8
Dec 19 14:25:12 localhost kernel: [ 4965.411256] CR2: 0000000000000048
Dec 19 14:26:11 localhost kernel: [ 5023.455769] SysRq : Keyboard mode set to system default
Dec 19 14:26:12 localhost kernel: [ 5024.951772] SysRq : Terminate All Tasks