drm/msm/sde: signal pageflip event after frame kickoff

sde_crtc module updates the event status before frame
kickoff and triggers the pageflip event on vsync callback.
That may lead to trigger pageflip event before actual
frame kickoff if there is pending vsync process. This
change updates the crtc event status after frame kickoff
and triggers the pageflip event in wait_for_commit done
to avoid race condition.

Change-Id: Ibfbef206495cbd66e2c401afaf4ba056439cf738
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>