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 <arnd@arndb.de>
Reported-by: Intel's 0-DAY
Fixes: 8d021d71 ("drm/nouveau/drm/nouveau: add a LED driver for the NVIDIA logo")
Signed-off-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>