[Intel-gfx] [PATCH] drm/vblank: Fixup and document timestamp update/read barriers

[shuang.he at intel.com]

Tested-By: Intel Graphics QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6213
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                                  276/276              276/276
ILK                                  302/302              302/302
SNB                                  318/318              318/318
IVB                                  341/341              341/341
BYT                                  287/287              287/287
HSW                                  395/395              395/395
BDW                                  318/318              318/318
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
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