drm: rcar-du: Convert LVDS encoder code to bridge driver

The LVDS encoders used to be described in DT as part of the DU. They now
have their own DT node, linked to the DU using the OF graph bindings.
This allows moving internal LVDS encoder support to a separate driver
modelled as a DRM bridge. Backward compatibility is retained as legacy
DT is patched live to move to the new bindings.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>