drm/msm/sde: remove frame done wait from sde_crtc

SDE crtc module waits for previous frame done
before it triggers a new frame. This is unnecessary
because sde encoder has similar wait already within
prepare_kickoff API call. This patch avoids extra
frame done wait and only flush the event thread
to trigger pending fences.

Change-Id: I45b6dcd5ec39491303fe29eaaf41988a68f48465
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>