drm/nouveau/secboot: start LS firmware in post-run hook

The LS firmware post-run hook is the right place to start said LS
firmware. Moving it here also allows to remove special handling in the
ACR code.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>