[Intel-gfx] [PATCH] drm/atomic-helpers: Properly avoid full modeset dance

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6066
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -1              276/276              275/276
ILK                                  303/303              303/303
SNB                                  304/304              304/304
IVB                                  339/339              339/339
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@minor-unsync-interruptible      PASS(3)      DMESG_WARN(1)PASS(1)
(dmesg patch applied)WARNING:at_drivers/gpu/drm/i915/i915_gem_evict.c:#i915_gem_evict_vm[i915]()@WARNING:.* at .* i915_gem_evict_vm+0x
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