drm: rcar-du: Add writeback support for R-Car Gen3

Implement writeback support for R-Car Gen3 by exposing writeback
connectors. Behind the scene the calls are forwarded to the VSP
backend.

Using writeback connectors will allow implemented writeback support for
R-Car Gen2 with a consistent API if desired.

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