drm: rip out drm_core_has_AGP

Most place actually want to just check for dev->agp (most do, but a
few don't so this fixes a few potential NULL derefs). The only
exception is the agp init code which should check for the AGP driver
feature flag.

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