[POWERPC] Fix sleep on some powerbooks

The PMU backlight code would kick in during sleep/resume even on
machines that use a different backlight method.  This breaks
sleep on some PowerBooks.

This fixes it by adding a flag to indicate whether the backlight
is controlled by the PMU, and testing that before trying to use
the PMU to turn off the backlight during sleep.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>