drm/msm/sde: trigger single frame_done evt for vid encoder

Commit 5db43498 ("drm/msm/sde: avoid frame_done
event trigger for idle scenario") supports frame_done
event merging with release and retire fence for
video mode. This works for single dsi display but it
causes issue for dual dsi display. Dual dsi display
needs two frame_done events in current design. The
second dummy frame_done event is generated by physical
video mode display. This patch simplifies the
frame_done event trigger for video mode by generating
single event. It aligns the logic with wb display.
Command mode display still waits for two pp_done
IRQs and triggers the frame_done interrupt when
both events are arrived.

Change-Id: I8d5a15d1d478001450b3e86a84f19e031f777d8d
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>