[Intel-gfx] [PATCH v3] drm/i915: Workaround to avoid lite restore with HEAD==TAIL

[shuang.he at intel.com]

Tested-By: Intel Graphics QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6205
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                                  276/276              276/276
ILK                 -2              302/302              300/302
SNB                                  318/318              318/318
IVB                                  341/341              341/341
BYT                                  287/287              287/287
HSW                                  395/395              395/395
BDW                                  318/318              318/318
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*ILK  igt at gem_fenced_exec_thrash@no-spare-fences-busy      PASS(3)      DMESG_WARN(1)PASS(1)
(dmesg patch applied)drm:i915_hangcheck_elapsed[i915]]*ERROR*Hangcheck_timer_elapsed...bsd_ring_idle at Hangcheck timer elapsed... bsd ring idle
 ILK  igt at gem_unfence_active_buffers      DMESG_WARN(1)PASS(1)      DMESG_WARN(1)PASS(1)
(dmesg patch applied)drm:i915_hangcheck_elapsed[i915]]*ERROR*Hangcheck_timer_elapsed...bsd_ring_idle at Hangcheck timer elapsed... bsd ring idle
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