drm/i915: report all active objects as busy

Incorporates a similar patch by Daniel Vetter, the alteration being to
report the current busy state after retiring.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>