drm: rcar-du: Expose the VSP1 compositor through KMS planes

On R-Car Gen3 SoCs the DU lost its ability to access memory directly and
needs to work in conjunction with the VSP to do so. This commit handles
the VSP internally to hide it from the user.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>