Fix login modeset

Ever since I moved to a new install of Manjaro SDDM always mirrors my monitors... no idea why that's the case but it causes a problem with kwin_wayland: KWin ignores the actual initial mode for rendering.

This patch intends to fix that by setting the correct mode for the waylandOutputDevice in AbstractWaylandOutput; kded then sets the resolution to what's wanted. I also added the mode index to the saved properties in the DrmBackend.