drm/mediatek: replace writeb() with mtk_dsi_mask()

The writeb() is unavailable in mt8173. Because the mt8173 dsi module
doesn't support 8bit mode access.

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