[AGPGART] intel_agp: fix G965 GTT size detect

On G965, I810_PGETBL_CTL is a mmio offset, but we wrongly take it
as pci config space offset in detecting GTT size. This one line patch
fixs this.

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