drm: rcar-du: Configure DPAD0 routing through last group on Gen3

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>