gpu: drm: sun4i_drv: 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: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>