drm/i2c: tda998x: avoid race when programming audio

Avoid a race between programming audio and an in-progress mode set.
A mode set is complex, and disables the ability to send infoframes
to the sink, and is disruptive to audio - we have to mute the audio
FIFO while doing a mode set.

If an attempt is made to start up the audio side, we will undo the
audio FIFO mute before the mode set has completed.

Move the lock so that we prevent audio interfering with an in-progress
mode set.

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>