[DRM plugin] Remember static kernel objects, amplify use of DrmCrtc

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.

Test Plan:
Login, logout, VT switching, connect and disconnect external monitor, energy
saving mode.

Reviewers: #kwin

Subscribers: kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5118