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drm/i915: Throw away the active object retirement complexity
Remove the accumulated optimisations that we have for i915_vma_retire and reduce it to the bare essential of tracking the active object reference. This allows us to only use atomic operations, and so will be able to avoid the struct_mutex requirement. The principal loss here is the shrinker MRU bumping, so now if we have to shrink, we will do so in much more random order and more likely to try and shrink recently used objects. That is a nuisance, but shrinking active objects is a second step we try to avoid and will always be a system-wide performance issue. The other loss is here is in the automatic pruning of the reservation_object when idling. This is not as large an issue as upon reservation_object introduction as now adding new fences into the object replaces already signaled fences, keeping the array compact. But we do lose the auto-expiration of stale fences and unused arrays. That may be a noticeable problem for which we need to re-implement autopruning. Signed-off-by:Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: