[Intel-gfx] [PATCH v2] drm/i915/skl: Fix up positive error code

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6037
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -2              269/269              267/269
ILK                 -1              303/303              302/303
SNB                                  304/304              304/304
IVB                                  338/338              338/338
BYT                                  287/287              287/287
HSW                                  362/362              362/362
BDW                                  310/310              310/310
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
 PNV  igt at gem_userptr_blits@coherency-sync      CRASH(1)PASS(3)      CRASH(2)
 PNV  igt at gem_userptr_blits@minor-unsync-normal      DMESG_WARN(1)PASS(1)      DMESG_WARN(1)PASS(1)
*ILK  igt at gem_seqno_wrap      PASS(3)      DMESG_WARN(1)PASS(1)
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