[Intel-gfx] [PATCH] drm/i915/bxt: Add Broxton steppings

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6015
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                                  274/274              274/274
ILK                                  303/303              303/303
SNB                                  303/303              303/303
IVB                 -1              342/342              341/342
BYT                                  287/287              287/287
HSW                                  362/362              362/362
BDW                                  308/308              308/308
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
 IVB  igt_gem_pwrite_pread_snooped-copy-performance      DMESG_WARN(1)PASS(3)      DMESG_WARN(1)PASS(1)
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