drm/nv50/pm: add support for pwm fan control

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nouveau_pm.c b/drivers/gpu/drm/nouveau/nouveau_pm.c
index 607e496..ee2872e 100644
--- a/drivers/gpu/drm/nouveau/nouveau_pm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_pm.c
@@ -167,8 +167,11 @@
 		}
 	}
 
-	if (pm->fanspeed_get)
-		perflvl->fanspeed = pm->fanspeed_get(dev);
+	if (pm->fanspeed_get) {
+		ret = pm->fanspeed_get(dev);
+		if (ret > 0)
+			perflvl->fanspeed = ret;
+	}
 
 	return 0;
 }