drm/msm/sde: flush master and slave interfaces in split display

Ensure that both the master and slave interface bits in the
interface flush register are updated in a split display
configuration using single control path.

Change-Id: I83cdb19d93f7d6e09efb59805f49dc7766ccf5ff
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>