drm: mxsfb: Use drm_panel_bridge

Replace the manual connector implementation based on drm_panel with the
drm_panel_bridge helper. This simplifies the mxsfb driver by removing
connector-related code, and standardizing all pipeline control
operations on bridges.

A hack is needed to get hold of the connector, as that's our only source
of bus flags and formats for now. As soon as the bridge API provides us
with that information this can be fixed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-3-laurent.pinchart@ideasonboard.com