drm/i915: Split the ringbuffers from the rings (3/3)

Manual cleanup after the previous Coccinelle script.

Yes, I could write another Coccinelle script to do this but I
don't want labor-replacing robots making an honest programmer's
work obsolete (also, I'm lazy).

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>