DTK audio: merge audio sample timers.

Instead of having the DVD code push samples to the mixer on a separate
timer, just use the existing streaming sample timer in the AI code.

This should make it much more clear how this code works for anyone reading
it.  Also fixes some small bugs; we can now properly pause audio, and
audio interrrupts should be a bit more accurate.
Audio mixer: add a TODO about sample rates.

Apparently the mixer wasn't expecting 48kHz input; this doesn't seem to
cause issues, but I'm not sure if it could.
Audio interface: add missing check for audio streaming.

We don't want to update samples if we aren't streaming audio.
Audio interface: add missing call to ADPCM InitFilter.