msm: mdss: check for split display flag during LM allocation

Command mode split display is only supported on DSPP0 and
DSPP1. The current software logic doesn't check for split
display flag causing mixer allocation failure for single DSI
command mode panels on LM-1. Add the check to take care
of this.

Change-Id: Ia8e8f2550c921b2d62ad05b1b99c8bb7e5f54f09
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>