agp/intel: Use pci_bus_address() to get GTTADR bus address

Per the Intel 915G/915GV/... Chipset spec (document number 301467-005),
GTTADR is a standard PCI BAR.

The PCI core reads GTTADR at enumeration-time.  Use pci_bus_address()
instead of reading it again in the driver.  This works correctly for both
32-bit and 64-bit BARs.  The spec above only mentions 32-bit GTTADR, but we
should still use the standard interface.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>