media: mtk-vcodec: fix leaked of_node references

The call to of_find_device_by_node 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-vcodec/mtk_vcodec_dec_pm.c:60:3-9: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 38, but without a corresponding object release within this function.
drivers/media/platform/mtk-vcodec/mtk_vcodec_dec_pm.c:63:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 38, but without a corresponding object release within this function.
drivers/media/platform/mtk-vcodec/mtk_vcodec_dec_pm.c:72:3-9: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 38, 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>