drm/nouveau/secboot: fix inconsistent pointer checking

We were returning PTR_ERR() on a NULL pointer, which obviously won't
work. nvkm_engine_ref() will return an error in case something went
wrong.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>