msm: mdss: Only use DSPP0 and DSPP1 for cmd mode split display

For command mode split display, due to hardware restrictions, only
the combination of DSPP0 and DSPP1 can be used.

Change-Id: I12feeb0807dd9a217a3f5540478c6d71fb4b8b80
CRs-fixed: 565993
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>