drm/msm/sde: avoid frame_done event trigger for idle scenario

Current sde video mode encoder triggers frame done event
without checking pending frame count in vblank_wait
API call. That may lead to multiple frame_done events
for last frame during encoder disable sequence. It can
be avoided by managing frame_done event trigger along
with retire fence because retire fence trigger always
checks the pending frame count. It also avoids the
frame_event callback from two different places
and reduced event thread workload.

Change-Id: I4bf0723f559c8c235bf56d8a676f6b6e8b6d6381
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>