drm: rcar-du: Constify node argument to rcar_du_lvds_connector_init()

The node passed as a pointer to the rcar_du_lvds_connector_init()
function is never modified, make it const.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>