drm/nouveau/therm: move thermal-related functions to the therm subdev

It looks scary because of the size, but I tried to keep the differences minimal.
Further patches will fix the actual "driver" code and add new features.

v2: change filenames, split to submodules

v3: add a missing include

v4: Ben Skeggs <bskeggs@redhat.com>
- fixed set_defaults() to allow min_duty < 30 (thermal table will
  override this if it's actually necessary)
- fixed set_defaults() to not provide pwm_freq so nv4x (which only has
  pwm_div) can actually work.  the boards using pwm_freq will have a
  thermal table entry to provide us the value.
- removed unused files

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