drm/mediatek: don't break dither->dsi0 link on mt8183

Cherry-pick fix from RITY kernel to fix dsi issue on i500

The dither->dsi0 path is hardcoded to use register (MMSYS+0xf68) but this register is actually at a different offset on mt8183, so we are wrongly overriding the DISP_RDMA 0_RSZ_OUT _SEL_IN register on MT8183 which break the DSI path.

This commit only update the dither->dsi0 path if a dsi0_sel_in_dither attribute is set into the mmsys_reg_data structure.

Signed-off-by: Fabien Parent fparent@baylibre.com