drm/nouveau/bios/fan: hardcode the fan mode to linear

This is an oversight that made use of the trip-point-based fan managenent on
cards that never expose those. This led the fan to stay at fan_min.

Fortunately, the emergency code would kick when the temperature would reach
90°C.

Reported-by: Tom Englund <tomenglund26@gmail.com>
Tested-by: Tom Englund <tomenglund26@gmail.com>
Signed-off-by: Martin Peres <martin.peres@free.fr>
Tested-by: Daemon32 <lnf.purple@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92126
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org