[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: 5699
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                                  353/353              353/353
ILK                                  353/353              353/353
SNB              +1                 400/422              401/422
IVB              +1-1              485/487              485/487
BYT                                  296/296              296/296
HSW              +1-1              507/508              507/508
BDW                                  401/402              401/402
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*SNB  igt_kms_flip_event_leak      NSPT(6, M35M22)      PASS(1, M35)
 IVB  igt_gem_pwrite_pread_snooped-pwrite-blt-cpu_mmap-performance      DMESG_WARN(9, M34M21)PASS(10, M4M34)      PASS(1, M34)
 IVB  igt_gem_storedw_batches_loop_secure-dispatch      DMESG_WARN(1, M34)PASS(9, M34M4)      DMESG_WARN(1, M34)
*HSW  igt_gem_pwrite_pread_display-copy-performance      PASS(7, M40M20)      DMESG_WARN(1, M40)
 HSW  igt_gem_pwrite_pread_snooped-pwrite-blt-cpu_mmap-performance      DMESG_WARN(2, M40)PASS(23, M40M20)      PASS(1, M40)
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