drm: rcar-du: Update Gen3 output limitations

The R-Car Gen3 DU utilises the VSP1 hardware for memory access. The
limits on the RPF and WPF in this pipeline are 8190x8190.

Update the supported maximum sizes accordingly.

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