drm/msm/sde: move crtc frame event handling to event thread

Currently crtc frame events are handled by display thread, which
might cause issues during suspend usecases. During suspend, the
encoder disable waits for pingpong done and sends the frame done
event to crtc which in turn reduces the frame_pending count. But
this might not execute until the display thread is done with suspend.
To avoid such cases and also to free up the display thread, move these
events to the event thread. Add completion logic to synchronizes the
frame_done event.

Change-Id: I2166d051e0cf68f7a7434df42263c43f588c094d
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>