drm/i915: Avoid using word legacy with ppgtt

The term legacy is subjective. Use 3lvl and 4lvl
where appropriate.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1488295691-9404-4-git-send-email-mika.kuoppala@intel.com