libdrm: drmCheckModesettingSupported: fix for FreeBSD

FreeBSD only support up to 10 GPUs not 16.

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