drm/nouveau/therm: force a minimum hysteresis on temperature alarm thresholds

This should avoid the situation where a user gets its kernel logs flooded when
temperature oscillates around a threshold with 0°C hysteresis.

This patch is just meant to fix broken vbios (as reported on a nv4e on
sysfs hwmon interface.

Signed-off-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>