drm: ->agp_init can't fail

Thanks to the removal of REQUIRE_AGP we can use a void return value
and shed a bit of complexity.

Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>