This function converts normalized doubles into an ALSA PCM format.

Instead of having per-format audio_signal_fill_* functions, we can only have
audio_signal_fill that outputs normalized doubles. Then in ALSA's playback
callback, we can simply use the new audio_convert_to function to fill the
buffer.

This makes the test code simpler and prevents code duplication when another
ALSA playback callback is implemented.

This adds a dependency of igt_audio over ALSA for the PCM format enum, but I
don't think this is a concern, since I don't see the point of using igt_audio
without igt_alsa. If this is an issue, it would always be possible to replace
ALSA's enum with our own in the future.

Signed-off-by: Simon Ser <simon.ser@intel.com>
Reviewed-by: Martin Peres <martin.peres@linux.intel.com>