[platforms/drm] Rework enablement and dpms switching

Summary:
This patch further refines output management.

We go now through AbstractWaylandOutput virtual functions to enable and
disable outputs.

Dpms changes and enablement switches use separate code paths at start in the
Drm backend code since they are similar but not the directly same. Common code
is shared though, functions are renamed accordingly.

Asserts have been put in place to better understand and check the control
flow. A seemingly unnecessary call to DrmOutput::pageFlipped on reactivation
after Vt switch has been removed to allow for that.

In future patches we need to look additionally at the legacy mode switching
code path which was and is still not working and better handling of the
current monitor Dpms state. For example a monitor being switched off is not
properly acted on and the workspace still expanded.

Test Plan:
With one and two monitors:

• Dpms off/on
• Vt switches
• Screen disable/enable

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Maniphest Tasks: T11459

Differential Revision: https://phabricator.kde.org/D23600