gpu-kms: Handle drmModeGetResources() failing

Avoid dereferencing the NULL return value if it fails. We still create the MetaGpu, but we treat it as if it has no outputs.

There is something really bad with this though. There is no fatal error, and so gnome-shell keeps on running even though there's nowhere for it to output to. This means that we don't fall back to X11 and so I get to see a nice blank screen.

I assume I need to make this fail somewhere else - any hints in that direction appreciated. (Or feel free to DIY if that's easier.)