drm/exynos: hdmi: move mode_fixup to drm common hdmi

Currently, mode_fixup code doesn't consider the limitations of mixer as it
is implemented inside the hdmi driver. Following fix, moves the mode_fixup
to common drm hdmi driver. To check the mode support, it calls both, mixer
and hdmi check_timing callbacks for a given resolution mode.

This patch is dependent on https://patchwork.kernel.org/patch/2176021/.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>