[Intel-gfx] [PATCH 4/4] drm/i915/skl: Implement do_mmio_flip for SKL

[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=273/273->273/273
PNV: pass/total=271/271->271/271
ILK: pass/total=269/271->271/271
IVB: pass/total=271/271->271/271
SNB: pass/total=271/271->271/271
HSW: pass/total=271/271->271/271
BDW: pass/total=271/271->271/271
-------------------------------------Detailed-------------------------------------
test_platform: test_suite, test_case, result_with_drm_intel_nightly->result_with_patch_applied
ILK: Intel_gpu_tools, igt_gem_concurrent_blit_gtt-bcs-gpu-read-after-write-forked, TIMEOUT->PASS
ILK: Intel_gpu_tools, igt_gem_concurrent_blit_gttX-bcs-gpu-read-after-write-forked, TIMEOUT->PASS