libdrm: drmGetDeviceNameFromFd: Add FreeBSD variant

Get the major/minor via fstat and after checking that this is a drm node
construct the full device node name using devname.
Note that we should be able to use fdevname to avoid calling fstat + devname
but for some reason it doesn't work on drm node (probably due to how the device
node are created in the linux compat code for drm on FreeBSD).

Signed-off-by: Emmanuel Vadot <manu@FreeBSD.org>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>