Thank you for this patch, I tested it on my 2017 ThinkPad X1 Yoga with openSuSE Tumbleweed (KWin 5.19, Kernel 5.8.4-1) and it works like a charm.
For enabling the monitor, I used QDBusViewer and called setCompositorBrightnessOutputNames() with eDP-1 manually.
Night mode also works out of the box. I'd love to see it upstream :)