OMAPDSS: DISPC: Add function to set channel in for writeback

Writeback can take input from either one of the overlays, or one of the overlay
managers. Add an enum which represents the channel_in for writeback, and maps
to the register field programming.

Add a function to configure channel in for writeback. This will be used later in
APPLY.

Signed-off-by: Archit Taneja <archit@ti.com>