drm/i2c: tda998x: avoid racy access to mode clock

Avoid a racy access to the mode clock by storing the current mode clock
during a mode set under the audio mutex.  This allows us to access it
from the audio path in a safe way.

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>