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.

[1]: https://github.com/swaywm/sway/issues/7457

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