gpu: host1x: Use for_each_available_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: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20240830073824.3539690-1-ruanjinjie@huawei.com