To get an image from KWin to the screen in the DRM pipeline we combine a CRTC, an encoder and a connector. These objects are static in the sense, that they represent real hardware on the graphics card, which doesn't change in a session. See here for more details: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html
Until now we used DrmOutput as the main representation for such an active rendering pipeline. I.e. it gets created and destroyed on hot plug events of displays. On the other side we had no fixed representation of the static kernel objects throughout the lifetime of KWin. This has several disadvantages:
* We always need to query all available static objects on an hot plug event.
* We can't manipulate the frame buffer of a CRTC after an output has been disconnected
* Adding functionality for driving multiple displays on a single CRTC (i.e. cloning) would be difficult
* We can't destroy the last frame buffer on display disconnect because the CRTC still accesses it and have therefore a memory leak on every display disconnect
This patch solves these issues by storing representations of all available CRTC and Connector objects in DrmBackend on init via DrmCrtc and DrmConnector instances. On an hotplug event these vectors are looped for a fitting CRTC and Connector combinations. Buffer handling is moved to the respective CRTC instance. All changes in overview:
* Query all available CRTCs and Connectors and save for subsequent hotplug events
* Fix logic errors in `queryResources()`
* Move framebuffers, buffer flip and blank logic in DrmCrtc
* Remove `restoreSaved()`. It isn't necessary and is dangerous if the old framebuffer was deleted in the meantime. Also could reveal sensitive user info from old session.