drm/ttm: fix ttm_bo_add_ttm(user) failure path

ttm_tt_destroy kfrees passed object, so we need to nullify
a reference to it.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: stable@kernel.org
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>