drm/exynos: mixer: use ratio precalculated in exynos_state

Common plane code already calculates and checks for supported scalling
modes, so additional code in mixer driver can be now removed.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>