gpu: drm: use for_each_endpoint_of_node()

We already have for_each_endpoint_of_node(), don't use
of_graph_get_next_endpoint() directly. Replace it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/87jzh3lnts.wl-kuninori.morimoto.gx@renesas.com