gpu: drm: omapdrm: dss-of: add missing of_node_put after calling of_parse_phandle

of_node_put needs to be called when the device node which is got
from of_parse_phandle has finished using.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>