drm/exynos/hdmi: simplify clock re-parenting

Driver tries to disable sclk_hdmi during re-parenting, to avoid possible
glitches. It is ineffective as the clock is used also by other devices (mixer).
Anyway driver works without disabling sclk_hdmi.

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