drm/mediatek: Fix mtk_cec_mask()

In current implementation, mtk_cec_mask() writes val into target register
and ignores the mask. After talking to our hdmi experts, mtk_cec_mask()
should read a register, clean only mask bits, and update (val | mask) bits
to the register.

Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220315232301.2434-1-miles.chen@mediatek.com/
Fixes: 8f83f268 ("drm/mediatek: Add HDMI support")
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Zhiqiang Lin <zhiqiang.lin@mediatek.com>
Cc: CK Hu <ck.hu@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_cec.c

@@ -85,7 +85,7 @@ static void mtk_cec_mask(struct mtk_cec *cec, unsigned int offset,
 	u32 tmp = readl(cec->regs + offset) & ~mask;
 
 	tmp |= val & mask;
-	writel(val, cec->regs + offset);
+	writel(tmp, cec->regs + offset);
 }
 
 void mtk_cec_set_hpd_event(struct device *dev,