glx: fix shared memory leak in X11

call XShmDetach to allow X server to free shared memory

Fixes: bcd80be4 "drisw/glx: use XShm if possible"
Signed-off-by: Ray Zhang <zhanglei002@gmail.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit b344e32c)