drm: rip out DRM_AGP_MEM and DRM_AGP_KERN

The <linux/agp_backend.h> header provides dummy functions and
fallbacks, so no need for screaming macros.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>