drm/exynos/hdmi: Simplify HPD gpio handling

GPIO is tested only in hdmi_detect, so there is no reason to set it in
other places and to preserve its value in context.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>