drm/msm/sde: support partial update on pingpong split topology

Add support for region of interest partial frame update when in
the pingpong split topology which uses a single layer mixer to
drive two DSI interfaces. This case requires special handling
due to the way a single layer mixer is shared, and the master
slave relationship between the two interfaces.

Change-Id: Ied4ec7101d1de800cb5803a5f7e95fd077fac350
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>