drm: exynos: hdmi: remove drm common hdmi platform data struct

exynos-drm-hdmi need context pointers from hdmi and mixer. These
pointers were expected from the plf data. Cleaned this dependency
by exporting i/f which are called by hdmi, mixer driver probes
for setting their context.

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