effects/kscreen: don't stop effect when fade out animation is done
requested to merge work/zamundaaa/cherry-pick-f6d14f1b44024af910718c6d5b4e29939914e871 into Plasma/5.26
The screen needs to stay black until it gets enabled again, otherwise slight timing differences can cause the screen to turn on full brightness again immediately before turning off.
BUG: 460902 (cherry picked from commit f6d14f1b)