drm/exynos/hdmi: add PHY power off signal handling

HDMI-PHY power off bit defaults to 0 in older HDMI versions.
In case of Exynos5433 it defaults to 1. To make code
consistent across all versions this bit is always unset/set in
power on/off sequences.

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