drm/arcpgu: fix device node leak

drm/arcpgu: fix device node leak

This function gets a device_node reference via
of_graph_get_remote_port_parent() and stores it in encoder_node, but never
puts that reference. Add it.

There used to be a of_node_put(encoder_node) but it has been removed by
mistake during a rework in commit 3ea66a794fdc ("drm/arc: Inline
arcpgu_drm_hdmi_init").

Fixes: 3ea66a794fdc ("drm/arc: Inline arcpgu_drm_hdmi_init")
Cc: stable@vger.kernel.org
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patch.msgid.link/20260402-drm-arcgpu-fix-device-node-leak-v2-1-d773cf754ae5@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>