drm: legacy: issue a NULL modeset on disable
Created by: Emantor
The DRM subsystem needs a NULL modeset for connectors which disappear from the system to disable the hardware pipes, otherwise the pixels get rendered but are sent nowhere.
The atomic backend does the equivalent by removing the properties and issuing a commit.
Fixes #1706 (closed)
Tested by using WLR_DRM_NO_ATOMIC=1 sway
and toggling my laptops internal eDP-1 display.