drm/exynos/mixer: replace direct cross-driver call with drm mode validation

HDMI driver called directly function from MIXER driver to invalidate modes
not supported by MIXER. The patch replaces the hack with proper .atomic_check
callback.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>