drm/nouveau/nouveau/led: prevent compiling the led-code if nouveau=y and leds=m

The proper fix would have been to select LEDS_CLASS but this can lead
to a circular dependency, as found out by Arnd.

This patch implements Arnd's suggestion instead, at the cost of some
auto-magic for a fringe feature.

Reported-by: Arnd Bergmann <[e-mail]>
Reported-by: Intel's 0-DAY
Fixes: 8d021d71b324 ("drm/nouveau/drm/nouveau: add a LED driver for the NVIDIA logo")
Signed-off-by: Martin Peres <[e-mail]>
Signed-off-by: Ben Skeggs <[e-mail]>

drivers/gpu/drm/nouveau/nouveau_led.h

···
 }
 
 /* nouveau_led.c */
-#if IS_ENABLED(CONFIG_LEDS_CLASS)
+#if IS_REACHABLE(CONFIG_LEDS_CLASS)
 int  nouveau_led_init(struct drm_device *dev);
 void nouveau_led_suspend(struct drm_device *dev);
 void nouveau_led_resume(struct drm_device *dev);