drm: rcar-du: Bump V3U to gen 4

V3U is actually gen 4 IP, like in V4H. Bump up V3U gen in the
rcar_du_r8a779a0_info.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>