ASoC: hdmi-codec: re-introduce mutex locking again

ASoC: hdmi-codec: re-introduce mutex locking again

The dai codec needs to ensure that on one dai is used at any time.
This is currently protected by bit atomic operation. With this change,
it done with a mutex instead.

This change is not about functionality or efficiency. It is done with
the hope that it help maintainability in the future.

Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20191206103542.485224-1-jbrunet@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>