drm/nouveau/therm: don't attempt fan control where PMU is already managing it

There's already a condition in place which attempts to detect this, but
since we've begun to require a PMU subdev even on boards where we don't
load a custom FW, it's become inaccurate.

This will prevent unnecessarily running a periodic fan update thread on
GP100 and newer, where we don't yet override the default PMU FW.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>