gpu: drm: exynos_hdmi: Move PHY logic into single function

Paring DT properties and getting PHY IO (memory mapped or I2C) in one
function.

Signed-off-by: Milo Kim <woogyom.kim@gmail.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>