drm/i915: fix up ring cleanup for the i830/i845 CS tlb w/a

It's not a good idea to also run the pipe_control cleanup.

This regression has been introduced whith the original cs tlb w/a in

commit b45305fc
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Dec 17 16:21:27 2012 +0100

    drm/i915: Implement workaround for broken CS tlb on i830/845

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=64610


Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>