AUO-K190x: add a 16bit truecolor mode

Some applications seem to have problems using the 8bit grayscale mode.
Therefore provide a 16bit truecolor mode which also gets converted to
the 4bit grayscale the display uses, when updating a region.

Signed-off-by: Heiko Stübner <heiko@sntech.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>