[Intel-gfx] Trivial priority scheduler

[Chris Wilson]

Now that the tree has caught up, we can now look at the next steps
towards a basic scheduler. No timeslicing, no preemption, no fairness,
we simply choose the highest priority request to execute next. (An
incoming high priority request will bump the priority on its
prerequisites so that it will execute at the earliest opportunity.)

Changes since the preview: a weekend of testing with lockdep and by
popular demand plug GuC into to a software scheduler.
-Chris