drm/exynos: add support for 'hdmi' clock

Mixed need to have hdmi clock enabled to properly perform power on/off
sequences, so add handling of this clock directly to the mixer driver.
Dependency between hdmi clock and mixer module has been observed on
Exynos4 based boards.

Suggested-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>