libdrm: drmGetDeviceNameFromFd: Always return /dev/dri/ node for FreeBSD

Since we now always returns the /dev/dri/ node for
drmGet<nodetype>DeviceNameFromFd, be consistant with the names returned
in drmGetDeviceNameFromFd.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Emmanuel Vadot <manu@FreeBSD.org>