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>