nouveau: backlight support for ppc laptops

Patch allows the backlight to be manipulated under gnome on apple powerpc
based NV30 machines.  It works fine on my powerbook, and should also work
for older NV17/NV18 machines.

Note that older powerpc specific tools (pbbuttonsd) have some problems with
this implementation (because the device is not yet there at the start time
of the daemon, and the code makes incorrect assumptions about the max
brightness values). However, IMHO these things can and should be addressed
in the daemon.

Some style/warning fixes applied by Stuart Bennett <stuart@freedesktop.org>