drm/i915: Rewrite some comments around RCU-deferred object free

Tvrtko noticed that the comments describing the interaction of RCU and
the deferred worker for freeing drm_i915_gem_object were a little
confusing, so attempt to bring some sense to them.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115205759.13884-1-chris@chris-wilson.co.uk