lima/gpir: Make lima_gpir_node_insert_child() useful

lima/gpir: Make lima_gpir_node_insert_child() useful

We weren't using this function before. The name is confusing, but it
changes the child while also fixing up the dependence link, if you don't
have access to it already. Or at least, I think that's what the
intention is, and what we'll need to change the branch condition in the
next commit. Adding a dependency between the new and old source doesn't
make any sense for this, and we also need to change the actual source.

Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4125>