Allow PCI VGA devices to be detached.

GPUs often have a VGA PCI class code and are probed/attached
by the VGA driver. Allow them to be detached so they can
be presented as passthru devices to VM guests.

Submitted by: mmacy
Reviewed by: jhb, imp, rgrimes
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D15269