[Intel-gfx] [PATCH] drm/i915: Fix an incorrect free rather than derefence issue.

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 5767
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -1              282/282              281/282
ILK                                  313/313              313/313
SNB                                  309/323              309/323
IVB                                  380/380              380/380
BYT                                  296/296              296/296
HSW                 -2              425/425              423/425
BDW                 -1              318/318              317/318
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*PNV  igt_gen3_render_linear_blits      PASS(5)      CRASH(1)PASS(1)
 HSW  igt_kms_flip_plain-flip-fb-recreate      TIMEOUT(2)PASS(1)      TIMEOUT(1)PASS(1)
 HSW  igt_kms_flip_plain-flip-fb-recreate-interruptible      TIMEOUT(2)PASS(2)      TIMEOUT(1)PASS(1)
*BDW  igt_gem_gtt_hog      PASS(8)      DMESG_WARN(1)PASS(1)
Note: You need to pay more attention to line start with '*'