drm/msm/sde: support partial update for dsc merge panel

Add support partial screen update for DSC merge panels. This
topology uses two pingpong and DSC encoders to drive a single
panel. It requires supporting left/right layer mixer swap in the
case where there is a right-only update.

Change-Id: Ie9c2d4990acbb6a4a65fddec2d8a42066bd3334e
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>