[Intel-gfx] [PATCH 2/2] drm/i915: Only enabled DRRS if PRS won't be enabled on this pipe.

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6043
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -1              269/269              268/269
ILK                                  303/303              303/303
SNB                                  304/304              304/304
IVB                                  338/338              338/338
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@coherency-sync      CRASH(1)PASS(3)      CRASH(1)PASS(1)
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