drm/exynos/hdmi: add driver data pointer to private context

The patch replaces duplicated driver data fields in private context with
pointer to driver data. It also simplifies driver data lookup code.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>