msm: mdss: enable synchronous flush only for video mode panels

Synchronous flush should not be used on command mode panels. If used,
as done in current implementation, it will only generate start signal
to left ctl path, and right ctl path will never receive start signal.
This leads to right half of the display to be blank. Fix this by
enabling synchronous flush only for video mode panels.

Change-Id: I9d950febcd0b5c046f6461dbb0a1f99208b93f06
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>