drm/msm/sde: add const alpha support for msmskunk

Add foreground and background blend layer constant alpha
programming sequence for msmskunk. Assign correct function to
the mixer HW interface based on HW revision.

Change-Id: I5af26fa56d6434adbea438ad31a1759d43b10b6d
Signed-off-by: abeykun <abeykun@codeaurora.org>