[Intel-gfx] [PATCH] drm/i915: use TAIL rather than ACTHD for synchronising reads

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                                  364/364              364/364
ILK              +1                 364/366              365/366
SNB                                  448/450              448/450
IVB                                  497/498              497/498
BYT                                  289/289              289/289
HSW                 -2              563/564              561/564
BDW                 -1              416/417              415/417
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
 ILK  igt_kms_flip_bcs-flip-vs-modeset-interruptible      DMESG_WARN(1, M26)PASS(2, M37M26)      PASS(1, M37)
*HSW  igt_gem_concurrent_blit_gpu-rcs-overwrite-source-forked      PASS(2, M40)      NO_RESULT(1, M40)
*HSW  igt_gem_concurrent_blit_gpuX-rcs-early-read-forked      PASS(2, M40)      NO_RESULT(1, M40)
*BDW  igt_gem_multi_bsd_sync_loop      PASS(2, M28M30)      DMESG_WARN(1, M30)
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