[Intel-gfx] [PATCH 2/2] drm/i915/skl: Don't try to disable DC6 if the DMC firwmare isn't loaded

[shuang.he at intel.com]

Tested-By: Intel Graphics QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6669
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
ILK                                  302/302              302/302
SNB                                  312/316              312/316
IVB                                  343/343              343/343
BYT                 -1              287/287              286/287
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*BYT  igt at gem_partial_pwrite_pread@reads-uncached      PASS(1)      FAIL(1)
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