virgl: Don't destroy resource while it's in use.

virgl: Don't destroy resource while it's in use.

This is the race condition: thread 1 check reference count of resource
and then find out out it's zero and then it begin to destroy it. Around
the same time, thread 2 gets the lock and get the resource from the hash
table and plan to use it. Then this resource gets destroyed while it's
still in use.

Signed-off-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9701>