venus: add vn_relax_init/_fini()

venus: add vn_relax_init/_fini()

Use a new calling contract so we can do pre/post-work around every ring-waiting
iteration. All looping uses of `vn_relax()` must now call `vn_relax_init()` and
`vn_relax_fini()` before/after their loop bodies.

Signed-off-by: Ryan Neph <ryanneph@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22036>