video: of: display_timing: Add of_node_put() in of_get_display_timing()

From code inspection it can be seen that of_get_display_timing() is
lacking an of_node_put().  Add it.

Fixes: ffa3fd21 ("videomode: implement public of_get_display_timing()")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722182439.44844-2-dianders@chromium.org