This fixes DPMS with KDE and radeonkms. Without this, the display would
freeze when the monitor is put into sleep state, and only resumes after
several dozens of minutes once the monitor is powered on again.
Tested by: Mathias Picker <Mathias.Picker@virtual-earth.de>
works with new generations of GPUs (IronLake, SandyBridge and
supposedly IvyBridge).
The driver is not connected to the build yet.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week