agp: Do not attach to Intel GEN6+

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