drm/mediatek: Adjust rdma fifo threshold calculate formula

The orginal formula will caused rdma fifo threshold config overflow
and no one could come out a solution for all SoC,
set threshold to 70% of max fifo size to make sure it will
not overflow, and 70% is a empirical vlaue.

Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_disp_rdma.c

@@ -162,10 +162,10 @@ void mtk_rdma_config(struct device *dev, unsigned int width,
 	/*
 	 * Enable FIFO underflow since DSI and DPI can't be blocked.
 	 * Keep the FIFO pseudo size reset default of 8 KiB. Set the
-	 * output threshold to 6 microseconds with 7/6 overhead to
-	 * account for blanking, and with a pixel depth of 4 bytes:
+	 * output threshold to 70% of max fifo size to make sure the
+	 * threhold will not overflow
 	 */
-	threshold = width * height * vrefresh * 4 * 7 / 1000000;
+	threshold = rdma_fifo_size * 7 / 10;
 	reg = RDMA_FIFO_UNDERFLOW_EN |
 	      RDMA_FIFO_PSEUDO_SIZE(rdma_fifo_size) |
 	      RDMA_OUTPUT_VALID_FIFO_THRESHOLD(threshold);