[AGPGART] intel_agp: don't load if no IGD and AGP port

After i915 chip, GMCH has no AGP port. Origin bridge driver in device
table will try to access illegal regs like APBASE, APSIZE, etc. This
may cause problem.

So mark them as NULL in the table, we won't load if no IGD got detect
and bridge has no AGP port.

Signed-off-by: Wang Zhenyu <zhenyu.z.wang@intel.com>
Signed-off-by: Dave Jones <davej@redhat.com>