Even though we've zeroed the PDE, the GPU may have cached the PD, so we
need to flush when deleting them.

Noticed while working on replacement MMU code, but a backport might be a
good idea, so let's fix it in the current code too.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org