I using KDE Desktop and got SDDM display manager by default. But I before use MATE Desktop and by default comming with LightDM. 

 

LightDM have support for VNC and I need that feature, here is procedure how to replace SDDM with LightDM (open console as root) :

apt install lightdm
apt install slick-greeter

Now reboot PC and can complete remove sddm package:

apt purge sddm