Translation:Was this movie scary?
So-called い-adjectives kind of include the auxiliary verb "to be" already and can be conjugated by themselves. E.g.「こわい」= "It is scary." The added です in case of neutral Japanese is for added politeness only. So when you change it to past tense, only the adjective itself changes: こわかった (= it was scary).
こわいでした or こわかったでした would both be incorrect.