mdss: mdp: Do not delay splash cleanup for split dsi

when split DSI is enabled do not delay the splash memory clean up.
because split dsi uses DSI0 & DSI1 for a single display hence there is
no need to delay splash clean up until last DSI interface.

Change-Id: Ibcf9884d82fc6b345df51e3e8650bfb45855d6b4
Signed-off-by: Rahul Sharma <sharah@codeaurora.org>