drm/radeon: add backlight quirk for hybrid mac

Mac laptops with multiple GPUs apparently use the gmux
driver for backlight control.  Don't register a radeon
backlight interface.  We may need to add other pci ids
for other hybrid mac laptops.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=65377



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org