On Gen3 SoCs DPAD0 routing is configured through the last CRTC group,
unlike on Gen2 where it is configured through the first CRTC group. Fix
the driver accordingly.

Fixes: 2427b303 ("drm: rcar-du: Add R8A7795 device support")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>