drm/i915: Stop retiring along engine

We no longer track the execution order along the engine and so no longer
need to enforce ordering of retire along the engine.

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/20190614164606.15633-2-chris@chris-wilson.co.uk