[Intel-gfx] [PATCH] drm/i915: Fix a use after free, and unbalanced refcounting

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 5797
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -2              277/277              275/277
ILK                                  313/313              313/313
SNB                                  309/309              309/309
IVB                                  382/382              382/382
BYT                                  296/296              296/296
HSW                                  425/425              425/425
BDW                 -1              318/318              317/318
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*PNV  igt_gem_fence_thrash_bo-write-verify-y      NO_RESULT(1)PASS(6)      FAIL(1)PASS(1)
 PNV  igt_gem_userptr_blits_coherency-sync      NO_RESULT(1)CRASH(5)PASS(6)      CRASH(1)PASS(1)
*BDW  igt_gem_gtt_hog      PASS(17)      DMESG_WARN(1)PASS(1)
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