drm/exynos/hdmi: replace all writeb with writel

Registers are 32-bit, even if only lower 8-bits are used.

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>