Add a specialized function for DRM drivers to register themselves.

Such drivers attach to a vgapci bus rather than directly to a pci bus. For
the rest of the LinuxKPI to work correctly in this case, we override the
vgapci bus' ivars with those of the grandparent.

Reviewed by: hselasky
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D11932