drm/msm/sde: update peripheral flush bit for dp dsc mode

Add peripheral flush bit in control path when dsc is active
for dp interfaces. This is required to flush the dp controller
logic to send pps packet in the current active frame transfer.

Change-Id: I82df60ea1fd00c6f68e128ff971553c1e2acee46
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>