drm/exynos: add mout_hdmi clock in hdmi driver to change parent

HDMI driver needs to configure the mout_hdmi mux clock to change
the parent between sclk_hdmiphy and sclk_pixel.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>