x86/gart: Tidy messages and add bridge device info

Print the AGP bridge info the same way as the rest of the kernel, e.g.,
"0000:00:04.0" instead of "00:04:00".

Also print the AGP aperture address range the same way we print resources,
and label it explicitly as a bus address range.

No functional change except the message changes.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>