drm/nouveau/therm: always initialize alarm_program_lock

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

Introduced in commit f825b1cd2235d3ea3365850bdc8397b3f671a218
"drm/nouveau/therm: implement support for temperature alarms"
(merged in 3.8).

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>