drm/exynos/hdmi: use optional regulator_get for hdmi-en

hdmi-en is an optional regulator so it should be better handled by
devm_regulator_get_optional call.

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