drm/nouveau/vm: perform a bar flush when flushing vm

Appears to fix the regression from "drm/nvc0/vm: handle bar tlb flushes
internally".

nvidia always seems to do this flush after writing values.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>