drm: nouveau: fix build when LEDS_CLASS=m

Fix build errors in nouveau driver when CONFIG_LEDS_CLASS=m and
CONFIG_DRM_NOUVEAU=y.
If LEDS_CLASS is enabled, DRM_NOUVEAU is restricted to the same
kconfig value as LEDS_CLASS.

drivers/built-in.o: In function `nouveau_do_suspend':
nouveau_drm.c:(.text+0x2030b1): undefined reference to `nouveau_led_suspend'
drivers/built-in.o: In function `nouveau_do_resume':
nouveau_drm.c:(.text+0x2034ca): undefined reference to `nouveau_led_resume'
drivers/built-in.o: In function `nouveau_drm_unload':
nouveau_drm.c:(.text+0x203a15): undefined reference to `nouveau_led_fini'
drivers/built-in.o: In function `nouveau_drm_load':
nouveau_drm.c:(.text+0x204423): undefined reference to `nouveau_led_init'

BTW, this line in Kbuild:
nouveau-$(CONFIG_LEDS_CLASS) += nouveau_led.o
does nothing when CONFIG_LEDS_CLASS=m and CONFIG_DRM_NOUVEAU=y.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Martin Peres <martin.peres@free.fr>
Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/029a1ec5-48ac-a3ce-3106-430e0f2584bb@infradead.org

drivers/gpu/drm/nouveau/Kconfig

 config DRM_NOUVEAU
 	tristate "Nouveau (NVIDIA) cards"
 	depends on DRM && PCI
+	depends on LEDS_CLASS || LEDS_CLASS=n
         select FW_LOADER
 	select DRM_KMS_HELPER
 	select DRM_TTM