drm/i915: Differentiate between LLC or snooped for the user

Rather than describing an object as either "snooped or LLC", we can do
better as we should know what machine we are running on!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>