drm: rcar-du: Allow DU groups to work with hardware indexing

The group objects assume linear indexing, and more so always assume that
channel 0 of any active group is used.

Now that the CRTC objects support non-linear indexing, adapt the groups
to remove assumptions that channel 0 is utilised in each group by using
the channel mask provided in the device structures.

Finally ensure that the RGB routing is determined from the index of the
CRTC object (which represents the hardware DU channel index).

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>