Currently it's not possible to control the brightness of the screen at the SDDM login screen after booting but first after logging in. It would be awesome it were possible to adjust the brightness whenever it feels convenient (just like in Windows 10). :)
Needs to be whitelisted in Powerdevil IIRC.
I doubt SDDM will ever get this feature. Let's retitle it to generic login screen, covering the upcoming plasma login manager.