[RFC] drm dynamic power off support

[Dave Airlie]

For optimus and powerxpress setups where we can explicitly power off
the GPU I'd like to do this under control of the driver. Now that I've
added X server support for secondary GPUs, it makes sense to start
powering them down more often if we can.

I've tested this code on two laptops, one intel/nouveau one intel/radeon
It works, powertop says I use 5-6W less power.

Caveats:
There is a race at X server start with platform probing code, if
the secondary device is off, we the wrong PCI info for it, I've
got a patch that works around this I'll send to the xorg-devel.

Audio seems to get screwed at least on one machine, we power up
and the alsa callbacks into snd_hda_intel get called but it can't
find the hw properly, need to investigate a bit further.

Dave.