drm/mediatek: Add pull-down MIPI operation in mtk_dsi_poweroff function

drm/mediatek: Add pull-down MIPI operation in mtk_dsi_poweroff function

In the dsi_enable function, mtk_dsi_rxtx_control is to
pull up the MIPI signal operation. Before dsi_disable,
MIPI should also be pulled down by writing a register
instead of disabling dsi.

If disable dsi without pulling the mipi signal low, the value of
the register will still maintain the setting of the mipi signal being
pulled high.
After resume, even if the mipi signal is not pulled high, it will still
be in the high state.

Fixes: 72d7fa4f9495 ("drm/mediatek: Add DSI sub driver")
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/1653012007-11854-5-git-send-email-xinlei.lee@mediatek.com/
Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Xinlei Lee <xinlei.lee@mediatek.com>
Reviewed-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>