drm: rcar-du: Support multiple sources from the same VSP

On R-Car H3 ES2.0, DU channels 0 and 3 are served by two separate
pipelines from the same VSP. Support this in the DU driver.

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