drm/i915: Kill off now unused gen6+ AGP code

v2: Accidently removed an ILK case in i9xx_setup (Nicely found by Chris)

CC: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by [v1] : Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>