drm/exynos/hdmi: simplify HDMI-PHY power sequence

Currently driver tries to set specific HDMI-PHY registers in three situations:
- before reset,
- before power off,
- after applying HDMI-PHY configuration.

First two cases seems to be unnecessary - register contents will be lost
anyway. The third case can be merged with HDMI-PHY configuration by fixing
the last byte of configuration data.

The patch has been tested with following platforms:
- exynos4210-universal_c210,
- exynos4412-odroidu3,
- exynos5422-odroidxu3.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>