drm/mediatek: Switch to for_each_child_of_node_scoped()

Introduce the scoped variant of the loop to automatically release the
child node when it goes out of scope, which is more robust than the
non-scoped variant, and accounts for new early exits that could be added
in the future.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20241011-mtk_drm_drv_memleak-v1-2-2b40c74c8d75@gmail.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>