glx: fix shared memory leak in X11

call XShmDetach to allow X server to free shared memory

Fixes: bcd80be49a "drisw/glx: use XShm if possible"
Signed-off-by: Ray Zhang <zhanglei002@gmail.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>

src/glx/drisw_glx.c

    if (pdp->ximage)
       XDestroyImage(pdp->ximage);
 
+   if (pdp->shminfo.shmid > 0)
+      XShmDetach(dpy, &pdp->shminfo);
+
    free(pdp->visinfo);
 
    XFreeGC(dpy, pdp->gc);