drm/nouveau/bl: Do not register interface if Apple GMUX detected

The Apple GMUX is the one managing the backlight, so there is no need for
Nouveau to register its own backlight interface.

v2: Do not split information message on two lines as it prevents from grepping
    it, as pointed out by Lukas Wunner

v3: Add a missing end-of-line character to the printed message
Signed-off-by: Pierre Moreau <pierre.morrow@free.fr>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_backlight.c

@@ -30,6 +30,7 @@
  * Register locations derived from NVClock by Roderick Colenbrander
  */
 
+#include <linux/apple-gmux.h>
 #include <linux/backlight.h>
 #include <linux/idr.h>
 
@@ -267,6 +268,11 @@ nouveau_backlight_init(struct drm_device *dev)
 	struct nvif_device *device = &drm->device;
 	struct drm_connector *connector;
 
+	if (apple_gmux_present()) {
+		NV_INFO(drm, "Apple GMUX detected: not registering Nouveau backlight interface\n");
+		return 0;
+	}
+
 	INIT_LIST_HEAD(&drm->bl_connectors);
 
 	list_for_each_entry(connector, &dev->mode_config.connector_list, head) {