drm/nouveau: Use for_each_child_of_node_scoped()

Avoids the need for manual cleanup of_node_put() in early exits
from the loop.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240830073654.3539640-1-ruanjinjie@huawei.com