Sddm in cauldron is now updated to the last release ( 0.0.9 ) and it is now working. Some work have been done for in the pam config file.
If you want to test it in mageia you will need to do :
systemctl disable prefdm.service && systemctl enable sddm.service
On next reboot it will not use prefdm anymore but sddm will start.
If you notice any probleme, please report your bug into Mageia Bugzilla.