drm: nouveau: Fix build warning seen if HWMON is undefined

Fix:

nouveau_pm.c: In function ‘nouveau_hwmon_init’:
nouveau_pm.c:703:24: warning: unused variable ‘therm’ [-Wunused-variable]

Introduced by commit 095f979a

 (drm/nouveau/pm: fix build with HWMON off) which
fixed a build error but introduced a build warning.

Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>