there are two cases to be handled here:
* normal
* software
the latter case requires env vars based on the frontend, and if a sw
device isn't found then init should fail
the former case should (in theory) just yolo the first device and assume
that's what the user wanted based on whatever env vars and layers are
in use
fixes#7508, #7132
maybe also affects #8152
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22184>
Mike Blumenkrantz
61451af5a2