drm/fsl-dcu: 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, but current code only
calls it at error path, fix it by adding it at correct code path.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>