drm/exynos: implement display-mode-check callback in mixer driver

drm/exynos: implement display-mode-check callback in mixer driver

This patch adds the implementation of check_timing callback in the mixer
driver. Based on the mixer version, correct set of restrictions will be
exposed by the mixer driver. A resolution will be acceptable only if passes
the criteria set by mixer and hdmi IPs.

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