[Misc] Move device options to a single place

[akx]

I noticed some of the devices available for vllm proper were not available for benchmarks.

This PR moves the triplicated list of available devices to a single place.

[github-actions]

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[akx]

The buildkite/fastcheck/pr/amd-openai-compatible-tool-use check error seems unrelated to this (a JSON decode error)...

[comaniac]

LGTM. I once found this was annoying too so thanks for this.

[akx]

@comaniac Test failures seem like unrelated flakes, and I don't have the required bits on Buildkite to retry a run :)