drm: rcar-du: Implement universal plane support

Explicitly create the CRTC primary plane instead of relying on the core
helpers to do so. This simplifies the plane logic by merging the KMS and
software planes.

Reject plane API operations on the primary planes for now, as that code
will anyway be refactored when implementing support for atomic updates.

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