drm/exynos/hdmi: convert container_of macro to inline function

Inline function is safer than macro, also the name has been changed to
be consistent with other inline function encoder_to_hdmi.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>