media: xilinx: vipp: Fix refcount leak in xvip_graph_dma_init

of_get_child_by_name() returns a node pointer with refcount
incremented, we should use of_node_put() on it when not need anymore.
Add missing of_node_put() to avoid refcount leak.

Fixes: df330515 ("[media] v4l: xilinx: Add Xilinx Video IP core")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>