drm/nv40/therm: increase the sensor's settling delay to 20ms

Based on my experience, 10ms wasn't always enough. Let's bump that
to a little more.

If this turns out to be insufficient-enough again, then an approach
based on letting the sensor settle for several seconds before starting
polling on the temperature would be better suited. This way, boot time
wouldn't be impacted by those waits too much.

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