drm/i915: Rename agp_type to cache_level

... to clarify just how we use it inside the driver and remove the
confusion of the poorly matching agp_type names. We still need to
translate through agp_type for interface into the fake AGP driver.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>