nouveau: allow invalidating coherent/persistent buffer backings

nouveau: allow invalidating coherent/persistent buffer backings

This is needed to support the core's usage of coherent buffers for
glVertex-style input. The reason why this was disallowed is that any
mappings will be invalidated. Let the state tracker worry about that,
and just reallocate when we're told.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Cc: mesa-stable@lists.freedesktop.org
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5276>