drm/ttm: Fix TTM object open regression

Commit drm/ttm: ttm object security fixes for render nodes introduced a
regression where, if a TTM object was opened multiple times from the same
open file, the caller would spin uninterruptibly in the kernel.

Fix this.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>