drm: rcar-du: lvds: Convert to DRM panel bridge helper

Replace the manual panel handling with usage of the DRM panel bridge
helper. This simplifies the driver, and brings support for
DRM_BRIDGE_ATTACH_NO_CONNECTOR as an added bonus.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham@ideasonboard.com> # On V3U
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>