backends/drm: make the placeholder screen as big as the last screen
The fixed size of 1080p makes windows change their size and place. To prevent that from happening, make the placeholder screen the same size as the last disconnected screen.
This is not a bullet proof solution (esp. in respect to multiple monitors), only a fast one. Ideally KWin should remember the window layout on a given monitor setup and restore it when reconnected.
BUG: 447419 FIXED-IN: 5.24