[Intel-gfx] [PATCH 2/2] drm/i915: remove unused has_dma_mapping flag

[shuang.he at intel.com]

Tested-By: Intel Graphics QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6754
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
ILK                                  302/302              302/302
SNB                                  312/316              312/316
IVB                                  343/343              343/343
BYT                                  287/287              287/287
HSW                                  380/380              380/380
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
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