drisw: Fix shared memory leak on drawable resize

XDestroyImage will mark the segment as to-be-destroyed, but it will
persist until we detach it, and we weren't doing so.

Cc: mesa-stable@lists.freedesktop.org
Gitlab: https://gitlab.freedesktop.org/mesa/mesa/issues/121


Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit c5a2ccec)