drm/i2c: tda998x: avoid race in tda998x_encoder_mode_set()

As priv->audio_params can now be changed at run time, we need to be more
careful about how we deal with a mode set.  We must take the audio lock
while checking if there's a valid audio configuration.

However, it's slightly worse than that - during mode set, we mute the
audio, and it must not be unmuted until we have finished the mode set.
It is possible that the audio side may start while a mode set is in
progress, so take the audio_mutex lock around the whole mode setting
procedure.

Tested-by: Jon Medhurst <tixy@linaro.org>
Acked-by: Jon Medhurst <tixy@linaro.org>
Tested-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>