drm/nouveau/therm: implement automatic fan management

v2: improved design but drops safety monitoring (to be in a later patch)
v3: fix locking and mode management
v4: gently fallback to the no-control mode when temperature cannot be got
    and use kernel-provided min/max macros
v5 (Ben Skeggs):
- rebased on my previous patches
v6: fix hysterisis management in trip-based auto fan management

This commit also forbids access to fan management to nvc0+ chipsets as
fan management is already taken care of my PDAEMON's default fw.

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