The i915 video driver doesn't depend on agp(4) anymore for Sandybridge
and later GPUs. Therefore, there is no need to attach agp(4) to those
devices.
While here, fix `agp_i965_res_spec` to include the aperture base for
GEN4 and GEN5.
Reviewed by: kib
Approved by: kib
Differential Revision: https://reviews.freebsd.org/D5586