backends/drm: fix failing atomic commits with disabled outputs
There were two issues:
- KWin did not handle allocations that fail too well. If it can't create any gbm surface for an output, it crashes. This is now mostly alleviated, but it will still crash if it has completely no surface for rendering (to be fixed with a future patch)
- the backend tried to create a gbm surface even for outputs that were disabled, which have no supported formats, which made all atomic tests fail if you had a disabled output. This is now fixed as well
Edited by Xaver Hugl