drm/i2c: tda998x: move and rename tda998x_encoder_set_config()

The naming of tda998x_encoder_set_config() is a left-over from when
TDA998x was a slave encoder.  Since this is part of the initialisation,
drop the _encoder from the name, and move it near tda998x_bind().

Tested-by: Robin Murphy <robin.murphy@arm.com>
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>