drm/msm/sde: add kthread_flush_work for crtc event thread

CRTC waits for frame_events of the previous frame to be
processed from the event thread before every kickoff. Though
the event thread is configured with realtime priority, it
might be taking longer duration to execute due to scheduling
delays and this results in frame_done timeouts as the commit
thread waits for only a specified duration. Add flush work
for event threads before the wait, to make sure the event
thread execution is completed.

Change-Id: I9a483e87daf8440a0bd90a76f3445c0a228c3856
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>