drm: rcar-du: Add R8A77965 support

The R8A77965 (M3-N) SoC provides RGB, HDMI and LVDS output.

This platform is unusual in that the RGB is connected to DU3 leaving DU2
unpopulated. This is reflected by the channels_mask accordingly.

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>