drm: rcar-du: Use drm_bridge_connector_init() helper

Use the drm_bridge_connector_init() helper to create a drm_connector for
each output, instead of relying on the bridge drivers doing so. Attach
the bridges with the DRM_BRIDGE_ATTACH_NO_CONNECTOR flag to instruct
them not to create a connector.

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