[platforms/drm] Early platform and outputs teardown
Since we now use in the backends the OutputDeviceInterface for output data
all access must be complete before the Wayland server goes down. For that
introduce a new function to prepare shutdown in the backends.
While at it also remove the output deletion, since they get deleted through
Qt's object system leading to crashes on double free.
Test Plan: Shutdown works without seg faults in the Drm backend.
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: zzag, kwin
Maniphest Tasks: T11459
Differential Revision: https://phabricator.kde.org/D23602