drm-shim: Let the driver choose to overwrite the first render node.

When I was writing drm-shim, I was focused on the v3d kmsro case -- use my intel device as the kmsro display device and add on a simulator-based v3d device that we could render with. But for the noop backends we use for shader-db, it's a lot more useful to just overwrite the first render node in the system so that you don't have to pass a -d argument.