nv50,nvc0: fix stream output target buffer leak

It manifests at exit as:
"WARNING: destroying GPU memory cache with some buffers still in use"

src/gallium/drivers/nv50/nv50_state.c

@@ -983,6 +983,7 @@ nv50_so_target_destroy(struct pipe_context *pipe,
    struct nv50_so_target *targ = nv50_so_target(ptarg);
    if (targ->pq)
       pipe->destroy_query(pipe, targ->pq);
+   pipe_resource_reference(&targ->pipe.buffer, NULL);
    FREE(targ);
 }
 

src/gallium/drivers/nvc0/nvc0_state.c

@@ -899,6 +899,7 @@ nvc0_so_target_destroy(struct pipe_context *pipe,
 {
    struct nvc0_so_target *targ = nvc0_so_target(ptarg);
    pipe->destroy_query(pipe, targ->pq);
+   pipe_resource_reference(&targ->pipe.buffer, NULL);
    FREE(targ);
 }