[Intel-gfx] [PATCH] drm/i915: Don't call intel_prepare_page_flip() multiple times on gen2-4

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                                  364/364              364/364
ILK              +1                 364/366              365/366
SNB                                  448/450              448/450
IVB                                  497/498              497/498
BYT                                  289/289              289/289
HSW                                  563/564              563/564
BDW                                  416/417              416/417
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
 ILK  igt_kms_flip_bcs-flip-vs-modeset-interruptible      DMESG_WARN(1, M26)PASS(4, M37M26)      PASS(1, M37)
Note: You need to pay more attention to line start with '*'