drm/imx/dcss: use drm_bridge_connector API

Make use of drm_bridge_connector API to have the connector initialized by the
display controller.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@nxp.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200731081836.3048-4-laurentiu.palcu@oss.nxp.com