drm: rcar-du: Don't select VSP1 sink on Gen3

The VSP1 sink selection through register DEFR8 is only available on Gen2
hardware. Skip it on Gen3.

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