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