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

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>