xf86drm: implement drmGetMinorNameForFD for non-sysfs

Implement drmGetMinorNameForFD for systems without sysfs by
adapting drm_get_device_name_for_fd() from the Mesa loader.

v2: use type parameter to select dev name instead of always
    using DRM_DEV_NAME

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>