LinuxKPI: return an address string in pci_name()

LinuxKPI: return an address string in pci_name()

amdgpu's virtual display feature uses pci_name() to match a module parameter
string, and the documentation shows an example of `0000:26:00.0` for the name.
In our case the name was just `drmn`, which is not actually unique across
devices.

The other consumers are wireless drivers, which will benefit from this
change.

Generate the expected string for pci_name() to return.

Related to:	https://github.com/freebsd/drm-kmod/issues/134
Sponsored by:	https://www.patreon.com/valpackett
Reviewed by:	bz, hselasky, manu (earlier)
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D34248