FifoPlayer: Make functions static

[tygyh]

No description provided.

[sepalani]

@tygyh
IMHO, I highly suggest that you stop opening (more) PRs in areas where it doesn't seem you're comfortable with. Especially, if it doesn't provide much value. The more PRs you open, the more likely you will have to deal with (merge/rebase) conflicts. On top of that, it might slow us down in the reviewing process (especially if you need to fix those conflicts multiple times).

Currently more than 10% of the open PRs are yours, see:
https://github.com/dolphin-emu/dolphin/issues?q=is%3Apr+is%3Aopen+author%3Atygyh

Here are suggestions of things you might want to focus on:

• Closing the PRs you opened that are: obsolete or don't make sense anymore.
• Changing incomplete PRs to draft if you still plan to work on them but they aren't currently ready (i.e. in a mergeable state).
• Updating the PRs you have conflicts with so they can get reviewed/merged.
• Addressing reviews in your existing PRs so they can reach a mergeable state.

Feel free to ping/remind us, if it's taking too long for some interesting PRs that were properly reviewed/approved to get merged. For instance, I'm think of #12938 which sounds like a nice change.