Not a pacman or update issue (you don't have a problem with the actual update), sounds like an issue with new sddm. Do you have the previous version still in your cache?
ls /var/cache/pacman/pkg | grep sddm
You need to have sddm-20170603-1-x86_64.pkg.tar.xz. If still in your cache, downgrade to it with:
sudo pacman -U /var/cache/pacman/pkg/sddm-20170603-1-x86_64.pkg.tar.xz
and reboot