backends/libinput: update screens on Workspace::outputsChanged

Using OutputBackend::outputsQueried can cause problems because it's emitted before output settings like enablement get adjusted to the new output configuration. It also doesn't react to output changes initiated by KScreen and is in the way of plans to use multiple output backends at the same time.

BUG: 461901 (cherry picked from commit cdeacdbd)

Backport of !3217 (merged)