drm/exynos/mixer: fix mode validation code

Mode limitation checked in mixer driver affects only older HW.
Mixer in Exynos542x has no such limitations. While at it patch changes
validation callback to recently introduced mode_valid which is more
suitable for the check. Additionally little cleanup is performed.

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>