[Intel-gfx] [PATCH] drm/i915: Make sure we invalidate frontbuffer on fbcon.

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 5923
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                                  282/282              282/282
ILK                                  308/308              308/308
SNB                                  307/307              307/307
IVB                                  375/375              375/375
BYT                                  294/294              294/294
HSW                                  385/385              385/385
BDW                                  315/315              315/315
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
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