[Intel-gfx] [PATCH] drm/i915: Fix frontbuffer false positve.

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 5820
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -1              281/281              280/281
ILK                                  308/308              308/308
SNB                 -1              326/326              325/326
IVB                                  380/380              380/380
BYT                                  294/294              294/294
HSW                                  387/421              387/421
BDW                 -1              316/316              315/316
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
 PNV  igt_gem_userptr_blits_minor-unsync-normal      DMESG_WARN(2)PASS(1)      DMESG_WARN(1)PASS(1)
 SNB  igt_kms_plane_plane-position-hole-pipe-B-plane-1      DMESG_WARN(12)PASS(3)      DMESG_WARN(1)PASS(1)
*BDW  igt_gem_gtt_hog      PASS(8)      DMESG_WARN(1)PASS(1)
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