drm/nouveau: enable hwmon support when both nouveau/hwmon are built as modules.

The nouveau hwmon temperature support currently only functions when hwmon is
compiled into the kernel. There's no reason why this shouldn't also work when
both hwmon and nouveau are modularised (as is the case with Slackware's stock
kernels).

Signed-off-by: Ken Milmore <ken.milmore@googlemail.com>
Reviewed-by: Martin Peres <martin.peres@ensi-bourges.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>