drm/nv40/therm: improve selection between the old and the new style

The condition to select between the old and new style was a thinko
as rnndb orders chipsets based on their release date (or general
chronologie hw-wise) and not based on their chipset number.

As the nv40 family is a mess when it comes to numbers, this patch
introduces a switch-based selection between the old and new style.

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