Fixes filename inconsistency for disabling GPU

[favorable-mutation]

Looks like this file is created and referred to in the enable/disable scripts as /etc/modprobe.d/disable-nvidia.conf, but in the systemctl service definition as /etc/modprobe.d/lock-nvidia.conf. This PR makes it consistent as disable instead of lock in the service definition as well.

[delta-one]

Thanks. Highly appreciated.