OMAPDSS: DPI: Add function to setup the channel for DPI

Add channel setup in DPI panel case.

Change-Id: Ib888c3666c2645a4ed0c3a67f4cc69aa876bc68d
Signed-off-by: Jonathan Bergsagel <jbergsagel@ti.com>
Signed-off-by: Dandawate Saket <dsaket@ti.com>