drm/exynos: Prepare/Unprepare HDMI subsystem clocks

Change the clk_enable/clk_disable calls in mixer and hdmi drivers into
clk_prepare_enable/clk_disable_unprepare, respectively.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>