Mesa's Vulkan WSI still uses wl_drm when modifiers aren't supported. This has been fixed in [1] but will take some time to be propagated to users. In the meantime, add a fallback. [1]: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4942
This will allow us to remove all of our EGL wl_drm support code and remove some weird stuff we need just for wl_drm support. In particular, wl_drm buffers coming from the EGL implementation can't easily be wrapped into a wlr_buffer properly.