[AGPGART] intel_agp: use table for device probe

Fixed issues noted by Christoph Hellwig, and I changed device table
scan a bit to allow the case that some models of graphics chips may
have same host bridge type. This type of chip will be added in the future.

This patch cleans up device probe function. Eric Anholt was the original author.
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Wang Zhenyu <zhenyu.z.wang@intel.com>
Signed-off-by: Dave Jones <davej@redhat.com>