nouveau runtime PM support (take two)

[Dave Airlie]

So I started investigating the kernel runtime PM support as per Alan's
suggestion. An API with lots of interface docs and not enough examples :-)

So I've mostly implemented autosuspend for nouveau with this, it
could still be pushed down a lot further into the code I suspect.

Also this doesn't actually turn the power switch off since runtime PM
currently have no hook to do that, so it only D3s the GPU.

Dave.