drm: rcar-du: lvds: Improve identification of panels

Dual-LVDS panels are mistakenly identified as bridges, this
commit replaces the current logic with a call to
drm_of_find_panel_or_bridge to sort that out.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>