sddm-theme: Avoid rendering invisible contents

An opacity of 0 will still render the items, particularly the blinking password field cursor will still be active.

This was addressed for the lock screen in 45e0a722 of plasma-workspace but not for the sddm-theme.

While the mentioned commit also set "enabled" to avoid some alleged cursor blinking, I could not reproduce this and just "visible" false seems to work fine.