Due to the lazy retirement semantics, even though we have unbound an
object, it may still hold onto an active reference. So in the debug code,
play safe.

v2: Export i915_gem_shrink() rather than opencoding it.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>