drm: rcar-du: Add num_rpf to struct rcar_du_device_info

Number of RPF's VSP is different on R-Car and RZ/G2L
 R-Car Gen3 -> 5 RPFs
 R-Car Gen2 -> 4 RPFs
 RZ/G2L -> 2 RPFs

Add num_rpf to struct rcar_du_device_info to support later
SoC without any code changes.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>