drm/i915: Defer accounting until read from debugfs

Simply remove our accounting of objects inside the aperture, keeping
only track of what is in the aperture and its current usage. This
removes the over-complication of BUGs that were attempting to keep the
accounting correct and also removes the overhead of the accounting on
the hot-paths.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>