If you use an external graphic card, the secondary device disappears,
which makes agpgart spew a message like

Detected an Intel i815 Chipset, but could not find the secondary device.

Which is ok, as we don't want to use it anyway.
This is confusing to users, as I've even had some people saying they
thought it was a hardware fault.
Signed-off-by: Dave Jones <davej@redhat.com>