drm/exynos/hdmi: clock code re-factoring

With incoming support for newer SoCs different set of clocks will be required,
depending on IP version. The patch prepares the driver for it.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>