st/vdpau: really block until surface is idle

pipe_screen::fence_finish with zero timeout returns quickly and
doesn't wait at all. Fix that, and also delete the fence afterwards,
so that QuerySurfaceStatus returns the right state later.

Addresses:
https://trac.videolan.org/vlc/ticket/9281
https://bugs.freedesktop.org/show_bug.cgi?id=68792

Reviewed-by: Christian König <christian.koenig@amd.com>

src/gallium/state_trackers/vdpau/presentation.c

@@ -325,7 +325,8 @@ vlVdpPresentationQueueBlockUntilSurfaceIdle(VdpPresentationQueue presentation_qu
    pipe_mutex_lock(pq->device->mutex);
    if (surf->fence) {
       screen = pq->device->vscreen->pscreen;
-      screen->fence_finish(screen, surf->fence, 0);
+      screen->fence_finish(screen, surf->fence, PIPE_TIMEOUT_INFINITE);
+      screen->fence_reference(screen, &surf->fence, NULL);
    }
    pipe_mutex_unlock(pq->device->mutex);