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>

drivers/gpu/drm/nouveau/nouveau_perf.c

@@ -304,8 +304,6 @@ nouveau_perf_init(struct drm_device *dev)
 	}
 
 	perf = nouveau_perf_table(dev, &ver);
-	if (ver >= 0x20 && ver < 0x40)
-		pm->fan.pwm_divisor = ROM16(perf[6]);
 
 	while ((perf = nouveau_perf_entry(dev, ++i, &ver, &hdr, &cnt, &len))) {
 		struct nouveau_pm_level *perflvl = &pm->perflvl[pm->nr_perflvl];