drm/i2c: tda998x: group audio functions together

Group the TDA998x audio functions together rather than split between
two different locations in the file, keeping like code together.

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>