[Intel-gfx] Track rpm wakerefs to fix bugs

[Chris Wilson]

If we keep a cookie for every time we acquire a wakeref, we can use that
to keep track of who holds a wakeref and more importantly who still
holds one at important junctures (when the rpm count unexpectedly goes
to zero!)

Culminating in a bugfix for Ironlake which CI has been tripping over for
many months, pretty much disabling fi-ilk-m540.
-Chris