drm/exynos: hdmi: Implement initialize op for hdmi

This patch implements the initialize callback in the hdmi and mixer
manager. This allows us to get rid of drm_dev in the drm_hdmi level and
track it in the mixer and hdmi drivers. This is one of the things
holding back the complete removal of the drm_hdmi layer.

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