media: mtk-vpu: fix leaked of_node references

The call to of_parse_phandle returns a node pointer with refcount
incremented thus it must be explicitly decremented after the last
usage.

Detected by coccinelle with the following warnings:
drivers/media/platform/mtk-vpu/mtk_vpu.c:477:1-7: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 464, but without a corresponding object release within this function.

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>