msm_fb: display: Unstage mixer staging on next commit for video use case

Unstage of mixer occurs on single vsync. For video playback case
on 8660 it is observed that sometime unstage does not complete
due to which flicker is seen during unset. Unstage of mixer is now done
on next commit only for video use case to avoid flicker.

Change-Id: I4939148989d49b3190142f858a40d2cf7617c7ba
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>