Introduce two new helpers, drm_agp_clear() and drm_agp_destroy() which
clear all AGP mappings and destroy the AGP head. This allows to reduce the
AGP code in core DRM and move it all to drm_agpsupport.c.
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>