drm/nouveau/therm: skip probing for devices not specified in thermal tables

Saves some time during driver load, as described by the relevant section[1]
of the DCB 4.x specification.

[1] https://nvidia.github.io/open-gpu-doc/DCB/DCB-4.x-Specification.html#_i2c_device_table



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