drm/mediatek: adjust dsi and mipi_tx probe sequence

mtk_mipi_tx is the phy of mtk_dsi.
mtk_dsi get the phy(mtk_mipi_tx) in probe().

So,  mtk_mipi_tx init should be ahead of mtk_dsi. Or mtk_dsi will
defer to wait mtk_mipi_tx probe done.

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>