drm/nouveau: Expose some BO usage flags to userspace.

This will be needed for Z compression and to take smarter placement
decisions.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>