monitor-utils: Check all libcamera v4l2 devices

The actual deduplication only checked the first device. Extend it to the full list, as intended.

Fixes: 848b6326 (monitor-utils: Rework camera deduplication code)

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@gkiagia thanks for landing !674 (merged) - and I'm sorry that I immediately have to open follow-up 🙈 Looking at the code again I figured I missed this multi-device case again - even though I'm sure I had it at some point, but lost it when moving back and forth between different solutions.

I quickly tested on affected devices that things keep working as intended and don't have any device that needs this change - but I guess the correctness is obvious enough.