st/nine: Use atomics for available_texture_mem

Resource dtor can be executed in the worker thread.
Use atomic to avoid threading safety issues.

CC: "17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Axel Davy <axel.davy@ens.fr>
Tested-by: James Harvey <lothmordor@gmail.com>
(cherry picked from commit bdf035ea6f784b90a6cbfd4ce16f3188ab15b2d3)
1 file changed