drm/i915: Discard the unused obj->last_fenced_ring

As we now never pipeline a fence update, obj->last_fenced_ring is always
the same as the obj->ring whenever obj->last_fenced_seqno is active, so
remove it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>