drm/i915: Track rcu_head for our idle worker

While our little rcu worker might be able to be replaced now by the
dedicated rcu_work, in the meantime we should mark up the rcu_head for
correct debugobjects tracking.

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/20181108092101.27598-2-chris@chris-wilson.co.uk