loader/dri3: Don't wait for fence of old buffer when re-allocating it

We only need to wait for the fence before drawing to a buffer, not
before reading from it.

This might avoid hangs when re-allocating the fake front buffer, similar
to the previous change. But I haven't seen any evidence that this was
actually happening in practice.

Tested-by: Olivier Fourdan <ofourdan@redhat.com>

src/loader/loader_dri3_helper.c

@@ -1777,7 +1777,6 @@ dri3_get_buffer(__DRIdrawable *driDrawable,
           && buffer) {
 
          /* Fill the new buffer with data from an old buffer */
-         dri3_fence_await(draw->conn, draw, buffer);
          if (!loader_dri3_blit_image(draw,
                                      new_buffer->image,
                                      buffer->image,