[AGPGART] intel_agp: fix device probe

This patch trys to fix device probe in two cases. First we should
correctly detect device if integrated graphics device is not enabled
or exists, like an add-in card is plugged. Second on some type of intel
GMCH, it might have multiple graphic chip models, like 945GME case, so
we should be sure the detect works through the whole table.

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