[Intel-gfx] [PATCH 7/7] drm/i915: don't reallocate the compressed FB at every frame

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 5775
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -3              281/281              278/281
ILK                                  313/313              313/313
SNB                                  309/309              309/309
IVB                                  382/382              382/382
BYT                                  296/296              296/296
HSW                                  426/426              426/426
BDW                 -1              318/318              317/318
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*PNV  igt_gen3_render_mixed_blits      PASS(1)      NRUN(1)
*PNV  igt_gen3_render_tiledx_blits      PASS(1)      NRUN(1)
*PNV  igt_gen3_render_tiledy_blits      PASS(1)      NRUN(1)
*BDW  igt_gem_gtt_hog      PASS(1)      DMESG_WARN(1)
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