drm/exynos: add display-mode-check operation to exynos_mixer_ops struct

This patch adds the display mode check operation to exynos_mixer_ops
in drm-common-hdmi. In Exynos SoCs, mixer IP can put certain restrictions
on the proposed display modes. These restriction needs to be considered
during mode negotiation, which happens immediately after edid parsing.

Both, mixer check-mode and hdmi check-timing callbacks are called one after
another and ANDed result is returned back.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>