drm/i915: Implement local atomic_state_free callback

As we use debugobjects to track the lifetime of fences within our atomic
state, we ideally want to mark those objects as freed along with their
containers. This merits us hookin into config->funcs->atomic_state_free
for this purpose.

This allows us to enable debugobjects for sw-fences without triggering
known issues.

Fixes: fc158405 ("drm/i915: Integrate i915_sw_fence with debugobjects")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161205142941.21965-4-chris@chris-wilson.co.uk