AUO-K190x: Use correct line length

Previously all functions that used the line length used xres directly,
thus hardcoding a 8bits per pixel value.

This patch calculates the correct line length according to the actual
bits per pixel value and changes all line length users to use the
calculated line length value.

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