drm/exynos: mixer: use crtc->state->adjusted_mode instead of crtc->mode

This patch replaces usage of crtc->mode with crtc->state->adjusted_mode
like it is already done in common plane code.

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>