MFC r259657,r264205,r264207:

r259657:

Add suspend/resume capabilities to the ATI backlight ppc driver.

With this, also 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.

r264205,r264207:

Fix the ATI backlight driver off/on handling. Now this driver works
correctly 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.

Approved by: re (marius)
Relnotes: yes (not suspend/resume, but the rest)