drm: rcar-du: Sort the DU outputs

Sort the DU outputs alphabetically, with the exception of the final
entry which is there as a sentinal.

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