drm/i2c: tda998x: introduce tda998x_audio_settings

Introduce a structure to hold the register values to be programmed while
programming the TDA998x audio settings.  This is currently a stub
structure, which will be populated in subsequent commits.

When we initialise this from the platform data, only do so if there is a
valid audio format specification.

Tested-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>