glx: fix memory leak related to __glXCloseDisplay()

Indeed, the unbind function is not called which could lead to an unbalanced refcount state for the "drawable" object.

For instance, with "piglit/bin/glx-fbo-binding -auto" while setting GALLIUM_REFCNT_LOG=refcnt.log.

Signed-off-by: Patrick Lerda patrick9876@free.fr