drm/nouveau/therm: always initialize alarm_program_lock

Fixes "BUG: spinlock bad magic" on module load for nva3+ cards.

Introduced in commit "drm/nouveau/therm: implement support for temperature
alarms".

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>