lima/gpir: Only try to place actual children

When picking a node to be scheduled, we try to schedule its children as
well. But we shouldn't try to schedule nodes which only have a fake
dependency on the original node, since this isn't the point of
scheduling children at the same time and can break some expectations of
the rest of the code.

Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>