drm/ttm: Fix spinlock imbalance

This imbalance may cause hangs when TTM is trying to swap out a buffer
that is already on the delayed delete list.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>