zink: rework choose_pdev (again)

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>
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