media: mtk-mdp: fix reference count on old device tree

of_get_next_child() increments the reference count of the returning
device_node. Decrement it in the check if we are using the old or the
new DTB.

Fixes: ba1f1f70 ("[media] media: mtk-mdp: Fix mdp device tree")
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Houlong Wei <houlong.wei@mediatek.com>
[hverkuil-cisco@xs4all.nl: use node instead of parent as temp variable]
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>