drm/i915: Only drop the batch-pool's object reference

The obj->batch_pool_link is only inspected when traversing the batch
pool list and when on the batch pool list the object is referenced. Thus
when freeing the batch pool list, we only need to unreference the object
and do not have to worry about the obj->batch_pool_link.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-5-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469530913-17180-4-git-send-email-chris@chris-wilson.co.uk