drm/mediatek: Separate mtk_hdmi_phy to an independent module

mtk_hdmi_phy is a part of mtk_hdmi module, but phy driver should be an
independent module rather than be part of drm module, so separate the phy
driver to an independent module.

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>