drm/nouveau/hwmon: fix crash on non-PCI platforms

Registration of the hwmon device will fail on non-PCI systems since
dev->pdev is NULL in that case. Use the more generic drm_device::dev
member that points to the same and is always set no matter the platform.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>