[Intel-gfx] [PATCH] drm/mst: use kref_get_unless_zero for looking

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
-------------------------------------Summary-------------------------------------
Platform: baseline_drm_intel_nightly_pass_rate->patch_applied_pass_rate
BYT: pass/total=249/348->248/348
PNV: pass/total=323/328->325/328
ILK: pass/total=328/330->330/330
IVB: pass/total=545/546->545/546
SNB: pass/total=551/556->550/556
HSW: pass/total=577/581->570/581
BDW: pass/total=435/435->434/435
-------------------------------------Detailed-------------------------------------
test_platform: test_suite, test_case, result_with_drm_intel_nightly(count, machine_id...)...->result_with_patch_applied(count, machine_id)...
BYT: Intel_gpu_tools, igt_gem_bad_reloc_negative-reloc-lut, PASS(4, M36) -> NSPT(1, M36)PASS(3, M36)
PNV: Intel_gpu_tools, igt_gem_concurrent_blit_gpu-bcs-early-read, PASS(6, M23M24) -> DMESG_WARN(1, M24)PASS(3, M24)
PNV: Intel_gpu_tools, igt_gem_linear_blits_normal, NSPT(1, M23)PASS(3, M24) -> PASS(4, M24)
PNV: Intel_gpu_tools, igt_gem_mmap_offset_exhaustion, DMESG_WARN(2, M23M24)PASS(2, M24) -> PASS(4, M24)
PNV: Intel_gpu_tools, igt_gem_unref_active_buffers, DMESG_WARN(1, M23)PASS(3, M24) -> PASS(4, M24)
ILK: Intel_gpu_tools, igt_kms_render_gpu-blit, DMESG_WARN(1, M26)PASS(3, M6) -> PASS(4, M6)
ILK: Intel_gpu_tools, igt_kms_flip_wf_vblank-vs-dpms-interruptible, DMESG_WARN(1, M26)PASS(3, M6) -> PASS(4, M6)
IVB: Intel_gpu_tools, igt_gem_bad_reloc_negative-reloc, NSPT(1, M4)PASS(3, M34M4) -> NSPT(2, M4)PASS(2, M4)
IVB: Intel_gpu_tools, igt_kms_plane_plane-position-covered-pipe-A-plane-1, TIMEOUT(1, M34)PASS(3, M4) -> PASS(4, M4)
SNB: Intel_gpu_tools, igt_gem_concurrent_blit_gpuX-bcs-overwrite-source-forked, PASS(4, M35M22) -> FAIL(1, M22)PASS(3, M22)
HSW: Intel_gpu_tools, igt_gem_bad_reloc_negative-reloc, NSPT(1, M39)PASS(3, M40M39) -> NSPT(2, M39)PASS(2, M39)
HSW: Intel_gpu_tools, igt_kms_cursor_crc_cursor-128x128-onscreen, PASS(4, M40M39) -> DMESG_WARN(2, M39)PASS(2, M39)
HSW: Intel_gpu_tools, igt_kms_cursor_crc_cursor-256x256-offscreen, PASS(4, M40M39) -> DMESG_WARN(2, M39)PASS(2, M39)
HSW: Intel_gpu_tools, igt_kms_cursor_crc_cursor-64x64-offscreen, DMESG_WARN(1, M39)PASS(3, M40M39) -> DMESG_WARN(2, M39)PASS(2, M39)
HSW: Intel_gpu_tools, igt_kms_cursor_crc_cursor-64x64-onscreen, DMESG_WARN(1, M39)PASS(3, M40M39) -> DMESG_WARN(1, M39)PASS(3, M39)
HSW: Intel_gpu_tools, igt_kms_cursor_crc_cursor-64x64-sliding, DMESG_WARN(1, M39)PASS(3, M40M39) -> DMESG_WARN(1, M39)PASS(3, M39)
HSW: Intel_gpu_tools, igt_kms_mmio_vs_cs_flip_setplane_vs_cs_flip, PASS(4, M40M39) -> DMESG_WARN(1, M39)PASS(3, M39)
BDW: Intel_gpu_tools, igt_gem_reset_stats_ban-bsd, PASS(4, M30) -> DMESG_WARN(1, M30)PASS(3, M30)