msm: mdss: Allocate DMA pipe for pan display path

On split dsi panels, allocate two DMA pipes for pan
display path. Currently same pipe is allocate which
is multi-rect instance of pipe.

Change-Id: Ic192be3bf8399b69cb6dd58b9aee31cac4bbeff2
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>