agp: Use pci_resource_start() to get CPU physical address for BAR

amd_irongate_configure(), ati_configure(), and nvidia_configure() call
ioremap() on an address read directly from a BAR.  But a BAR contains a
bus address, and ioremap() expects a CPU physical address.  Use
pci_resource_start() to obtain the physical address.

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