drm: rcar-du: Simplify encoder registration

drm: rcar-du: Simplify encoder registration

Before the driver fully moved to drm_bridge and drm_panel, it was
necessary to parse DT and locate encoder and connector nodes. The
connector node is now unused and can be removed as a parameter to
rcar_du_encoder_init(). As a consequence rcar_du_encoders_init_one() can
be greatly simplified, removing most of the DT parsing.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>