Some ATI-based PowerBooks use the string 'mnca' in the backlight controller
device tree entry, so account for this and don't use nVidia when it's not an
nVidia device.
MFC after: 3 weeks
with the ATI Radeon 9700 in the PowerBook G4 1.67GHz.
Code shamelessly taken in spirit from the radeonkms driver, which I hope will
make this driver redundant in the future.
MFC after: 2 weeks
With this, also shut shut off the display (DPMS-style) and disable the clocking
when the backlight level is set to 0. This is taken from the radeonkms driver
(radeon_legacy_encoders.c) which doesn't yet support PowerPC, and won't for a
while, as it's missing full AGP support.
