output-layout: fix missing global for outputs with a custom mode

Before we were populating wlr_output.current_mode with a generated fixed mode when a custom mode was committed in the DRM backend. But that's no longer the case: now a custom mode behaves the same under the DRM backend and other backends.

wlr_output_layout was still assuming that an output without a current_mode was disabled. Fix that assumption.

Fixes: 5567aefb ("backend/drm: Don't add pollute fixed modes list with custom modes")
Closes: #3743 (closed)