drm/nouveau/drm/nouveau/led: prevent a possible use-after-free

If the led class registration fails, we free drm->led but do not reset
it to NULL, which means that the suspend/resume/fini function will act
as if everything went well in init() and will likely crash the kernel.

This patch adds the missing drm->led = NULL.

Reported-by: Emmanuel Pescosta <emmanuelpescosta099@gmail.com>
Signed-off-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>