A NV49 appeared a while back that was using the "nv41 style" pwm registers,
rather than the "nv40 style" ones my board is using.  This disproves the
previous theory that the pwm controller choice is chipset-specific.

So, after looking at a bunch of vbios images it appears that the next viable
theory is that we should select the pwm controller to use based on the gpio
line the fan is tied to, just like we do on nv50.

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