disp: msm: sde: flush crtc event thread before idle notification

Flush sde crtc event thread before idle notification
to make sure that pending frame count is zero. This
allows sde encoder module to trigger power collapse
during pm_suspend scenario.

Bug: 146848315
Change-Id: Ic65a76273417c567c330e970c97183e5c0f4ad17
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>