backend/drm: Skip multi-gpu outputs in backend tests

Our multi-gpu path currently needs to blit a buffer in order to have a primaryfb to add to the commit. This is expensive, and we skip it entirely during test commits. This in turn also means that we skip tests commits entirely for such outputs, outside our own basic tests.

Backend-wide commits missed this check, and tried to perform test commits for multi-gpu outputs despite no primaryfb having been attached, making them always fail. Add the same exception as we have in the per-connector commit-test.