drm/exynos/mixer: abstract out output mode setup code

Mode setup code is called from video plane update and mixer plane update.
Let's group it together in mixer_commit function like in case of other
Exynos CRTCs.

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