nouveau optimus dynamic power off

[Dave Airlie]

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>