nouveau optimus dynamic power off

I finally got back to debugging the HDMI audio interactions that stopped me last time, This adds switcheroo + nouveau + hda_intel support for turning off the secondary GPU in optimus laptops, saving a lot of power. The GPU should come back on for things like lspci on the devices or for DRI_PRIME= or X starting up. Signed-off-by: Dave Airlie <airlied at redhat.com>