drm/msm/sde: enable ping pong split support

Add support for using a single layer mixer to drive two video
or command mode interfaces. This configuration frees up one of
the layer mixer paths for other use cases/displays.

Change-Id: I3a0ca07e37f121bf56ea8984c0cca25af7bc647e
Signed-off-by: Clarence Ip <cip@codeaurora.org>