radv: Use correct watermark for early loop exit.

radv: Use correct watermark for early loop exit.

The previous check assumed the stack starts at offset=0, which isn't
necessarily true for ray queries.

Note that this didn't cause correctness issues, just made an optimization
not apply. Found when I accidentally made this load-bearing in a
refactor.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20265>