[Intel-gfx] [RFC] Removing struct_mutex from around requests

[Chris Wilson]

The paint is not yet dry on this, and I don't expect it to pass BAT.
(Trybot has been down so unable to precheck for silly bugs.) There is
one race in i915_active.retire that eludes me (enabling kasan and
friends hides the race, making debugging harder than it should be!) and
I am sure that there are plenty more as this is one scary patchset.

The first 14 are pretty benign and desperately want review.
-Chris