nouveau: avoid double-emitting fence

The act of ensuring that there is space can cause a flush to happen,
which will emit the current screen fence. If that is the fence we're
trying to wait on, then it will have been emitted as a result of doing
the PUSH_SPACE. Don't attempt to emit it a second time.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Fixes: 8053c9208f (nouveau: avoid emitting new fences unnecessarily)
Cc: mesa-stable@lists.freedesktop.org

src/gallium/drivers/nouveau/nouveau_fence.c

@@ -192,7 +192,11 @@ nouveau_fence_wait(struct nouveau_fence *fence)
 
    if (fence->state < NOUVEAU_FENCE_STATE_EMITTED) {
       PUSH_SPACE(screen->pushbuf, 8);
-      nouveau_fence_emit(fence);
+      /* The space allocation might trigger a flush, which could emit the
+       * current fence. So check again.
+       */
+      if (fence->state < NOUVEAU_FENCE_STATE_EMITTED)
+         nouveau_fence_emit(fence);
    }
 
    if (fence->state < NOUVEAU_FENCE_STATE_FLUSHED)