[Intel-gfx] [PATCH] drm/i915: Don't dereference NULL plane while setting up scalers

[shuang.he at intel.com]

Tested-By: Intel Graphics QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6734
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
ILK                                  302/302              302/302
SNB                                  312/316              312/316
IVB                                  345/345              345/345
BYT                 -1              289/289              288/289
HSW                                  382/382              382/382
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*BYT  igt at gem_partial_pwrite_pread@reads      PASS(1)      FAIL(1)
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