amd64-agp: Probe unknown AGP devices the right way

The current initialisation code probes 'unsupported' AGP devices
simply by calling its own probe function.  It does not lock these
devices or even check whether another driver is already bound to
them.

We must use the device core to manage this.  So if the specific
device id table didn't match anything and agp_try_unsupported=1,
switch the device id table and call driver_attach() again.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>