drm: rcar-du: Enable alpha property on primary planes

The hardware supports alpha on all planes, and using it on the primary
plane can be useful. Don't restrict the alpha property to overlay
planes.

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