[Intel-gfx] [PATCH] drm/i915: Only dither on 6bpc panels

[shuang.he at intel.com]

Tested-By: Intel Graphics QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 7155
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
ILK                 -1              302/302              301/302
SNB                                  315/315              315/315
IVB                                  336/336              336/336
BYT                 -3              283/283              280/283
HSW                                  378/378              378/378
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*ILK  igt at kms_flip@wf_vblank-vs-modeset-interruptible      PASS(1)      DMESG_WARN(1)
*BYT  igt at gem_partial_pwrite_pread@reads-uncached      PASS(1)      FAIL(1)
*BYT  igt at gem_persistent_relocs@forked-interruptible      PASS(1)      FAIL(1)
*BYT  igt at gem_tiled_partial_pwrite_pread@reads      PASS(1)      FAIL(1)
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