Fix start on FreeBSD

Simply return primary gpu node when primary render node not found. Refer to: https://github.com/YaLTeR/niri/blob/c8eea8ee9ddf795da8cb356599f17e2dc7cfea20/src/backend/tty.rs#L315-L320 <img width="1920" height="1080" alt="20251025_03h59m07s_grim" src="https://github.com/user-attachments/assets/a2b1d251-6e23-47f3-8b07-2a177afa98ce" />