loader: remove final sysfs codepath in loader_get_device_name_for_fd()

loader: remove final sysfs codepath in loader_get_device_name_for_fd()

Effectively everyone with actual hardware and/or requesting the
"device_name" requires a working libdrm. Thus they could/should already
be using the (now only) codepath.

Apart from the code simplification, we can slim down our configure.ac
even further. But that will be done in separate patch(es).

Cc: Gary Wong <gtw@gnu.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>