fbdev: nvidia: use for_each_child_of_node() macro

Use for_each_child_of_node() macro instead of open coding it.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200916062123.190636-1-miaoqinglang@huawei.com