Boot-issue: Overlapping I/O regions and display adapters in -current

Hi,

It appears that overlapping I/O regions can trigger a bug on display adapters, 
which I've tried to fix here:

http://svn.freebsd.org/changeset/base/233662

I'm not an expert in this area, though if anyone has any better suggestions, I 
am willing to try that.

Sympthom: ACPI enabled i386 kernel works, ACPI enabled amd64 kernel stops 
during boot due to invalid CPU instruction exactly after disabling BAR on AGP 
device.

--HPS