drm/msm/sde: fix errors with pp-split video mode

Current driver flushes the bits for both interfaces
and triggers the frame updates in the case of pp-split.
This is not expected, given that in pp-split only the
master interface has the full control of the updates
and causes multiple errors to be thrown by the driver.
Fix this problem by avoiding the flush of the slave
interface as well as the frame update notification.

Change-Id: Ia95db70326f80128b6e68dab123930006b0af71b
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>