[platforms/drm] Properly handle case that mapping the blank buffer fails

Summary:
If it's not possible to create a buffer to blank the output, KWin won't
be able to render to the output. In that case the output should not be
added to the list of outputs.

To support this DrmOutput::blank and DrmOutput::init return bool to
indicate whether they succeeded. DrmBackend handles this situation and
doesn't add the output to the list of outputs if init failed.

If after init there are no outputs KWin is in a state where it won't
be functional. Thus the platform emits the initFailed signal to
terminate.

BUG: 365242
FIXED-IN: 5.7.2

Test Plan: No hardware to reproduce the condition

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2135