[PiSink] Set hdmi stream channels when in passthrough mode

The GPU needs to know when in passthrough mode to set the number of channels
in the AV info frame to "refer to stream", so let it know.

This fixes AC3/DTS passthrough with Pi Sink. (Needs recent firmware).
Merge pull request #4148 from popcornmix/streamchannels

[PiSink] Set hdmi stream channels when in passthrough mode