backend: make wlr_backend_autocreate() fail when DRM is missing
When we change the required dependencies for the DRM backend, Meson might auto-disable the backend for users missing the new requirements. This results in confused users 1 because they don't notice the "drm-backend: NO" line burried in the Meson logs, and then get a black screen when starting the compositor.
Update wlr_backend_autocreate() to refuse to create a backend with only libinput (without DRM).
Users really wanting to start their compositor with a libinput backend and without a DRM backend can manually set WLR_BACKENDS.