[PATCH] conf: vc4-hdmi: use a proper hdmi pcm, fix broken default pcm

[popcorn mix]

Matthias Reichl wrote:
> Userspace expects to see a HDMI pcm, so remove the somewhat broken
> front and iec958 pcms and add a proper hdmi pcm instead.
>
> The hdmi pcm supports HD/HBR audio passthrough and relays the IEC958
> status bits on to the kernel driver so it can switch to HBR audio
> packets if needed. The control hook is marked optional because kernels
> before 5.14 didn't expose the IEC958 controls.
>
> The default pcm never worked as dmix doesn't support the iec958 format.
> So drop dmix, only use plug and softvol, and use the hdmi pcm for
> iec958 formatting.

I can confirm the upstream cards/vc4-hdmi.conf is not usable,
and RPiOS ships a patched version (which was an earlier version of this patch).

I've tested this version with RPiOS bullseye and all seems good.
I could play stereo and multi-channel PCM from VLC.
I could also play DTS, DTS-HD and TrueHD formats as passthrough using kodi.

It would be good if we can switch from a custom version of vc4-hdmi.conf
to an upstream version in RPiOS when this is merged.

Tested-by: Dom Cobley <popcornmix at gmail.com>