drm/i2c: tda998x: add support for pixel repeated modes

TDA998x has no support for pixel repeated modes, and the code notes this
as a "TODO" item.  The implementation appears to be relatively simple,
so lets add it.

We need to calculate the serializer clock divisor based on the TMDS
clock rate, set the repeat control, and set the serializer pixel
repeat count.  Since the audio code needs the actual TMDS clock,
record that.

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