drm/i915: Remove one very outdated comment

Comment disagrees with the code which has changed a lot since
it was documented.

Note that the logic to remove -EIO handling was dropped in

commit 1488fc08
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Apr 24 15:47:31 2012 +0100

    drm/i915: Remove the deferred-free list

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>