backends/drm: prevent accessing the pipeline when it's already removed

This should fix https://crash-reports.kde.org/share/issue/2229c3a1957b46c0997fb62cbd0a0e94/

For Plasma 6.6, I'm confident that we'll have a longer term fix - my display mirroring patches make it so the compositor looks at the backend adding and removing outputs, not just at Workspace.