drm/mediatek: Move tz_disabled from mtk_hdmi_phy to mtk_hdmi driver

tz_disabled is used to control mtk_hdmi output signal, but this variable
is stored in mtk_hdmi_phy and mtk_hdmi_phy does not use it. So move
tz_disabled to mtk_hdmi where it's used.

Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Tested-by: Frank Wunderlich <frank-w@public-files.de>