drm/nouveau: Don't disable polling in fallback mode

When a fan is controlled via linear fallback without cstate, we
shouldn't stop polling.  Otherwise it won't be adjusted again and
keeps running at an initial crazy pace.

Fixes: 800efb4c ("drm/nouveau/drm/therm/fan: add a fallback if no fan control is specified in the vbios")
Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1103356
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107447


Reported-by: Thomas Blume <thomas.blume@suse.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>