drm/i915: Initialise the obj->rcu head

Make the rcu_head known to the system, in particular for debugobjects.
And having declared it for debugobjects, we need to tidy up afterwards.

v2: mark the obj->rcu as being destroyed when we reuse its location for
the freed list.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108691


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181109090311.15321-1-chris@chris-wilson.co.uk