The problem I see with trip-points is that those allow to set a fixed PWM value
when the sensors detect a certain temperature. The nouveau driver instead
raises the fan speed "continuously" after a certain temperature is reached
(would we need infinite trip-points for that?). 
Please correct any error in my understanding here :)

Maybe we can use the trip points as a start/end point and then move
autonomously disregarding the point[0-*]_pwm value?

For now, if you agree, I will move the sysfs values as suggested (and change
the documentation), hopefully before/during next weekend. 
So this bug will still have an almost "clean" patch for anyone (included me)
who want it.
Then I will ask around in the lm-sensors mailing list (is it the correct one?).
Hopefully someone can point me to a better solution.

Thanks for the review! :)