drm/msm/mdp5: Allocate CTL0/1 for dual DSI single FLUSH

This change takes advantage of a HW feature that synchronize
flush operation on CTL1 to CTL0, to keep dual DSI pipes in
sync.

Signed-off-by: Hai Li <hali@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>