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 Cc: mesa-stable@lists.freedesktop.org