[Intel-gfx] Vlv punit w/a (take two)

[Chris Wilson]

I've incorporated Ville's feedback that this is highly unlikely to be a
general problem and tied the w/a to only the Valleyview punit. I kept
it reasonably open just in case we need to extend it, and for an
interface that makes for locking all sideband access convenient. (If we
make punit access very expensive, we don't want to be doing it too
often!)
-Chris