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 <firstname.lastname@example.org>