ttm_tt_destroy kfrees passed object, so we need to nullify
a reference to it.
Signed-off-by: NMarcin Slusarz <marcin.slusarz@gmail.com>
Cc: stable@kernel.org
Reviewed-by: NThomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>