disp: msm: sde: Add flush support for DSPP SB

Extend unified flush bit support to control new DSPP
SB LUTDMA bit.

Change-Id: Iba941a4bcd140ceb88e49ab83700c4baef804e0f
Signed-off-by: Ping Li <pingli@codeaurora.org>
Signed-off-by: Christopher Braga <cbraga@codeaurora.org>