drm: rcar-du: Store V4L2 fourcc in rcar_du_format_info structure

The mapping between DRM and V4L2 fourcc's is stored in two separate
tables in rcar_du_vsp.c. In order to make it reusable to implement
writeback support, move it to the rcar_du_format_info structure.

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