drm/exynos: mixer: simplify mixer_cfg_rgb_fmt()

Convert if-statements to switch statement. Removes
duplicated code.

Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>