drm/mediatek: dsi: Use mipi_dsi_pixel_format_to_bpp() helper function

Instead of open coding, use the mipi_dsi_pixel_format_to_bpp() helper
function from drm_mipi_dsi.h in mtk_dsi_poweron() and for validation
in mtk_dsi_bridge_mode_valid().

Note that this function changes the behavior of this driver: previously,
in case of unknown formats, it would (wrongly) assume that it should
account for a 24-bits format - now it will return an error and refuse
to set clocks and/or enable the DSI.

This is done because setting the wrong data rate will only produce a
garbage output that the display will misinterpret both because this
driver doesn't actually provide any extra-spec format support and/or
because the data rate (hence, the HS clock) will be wrong.

Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240215085316.56835-10-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

Author: AngeloGioacchino Del Regno

drivers/gpu/drm/mediatek/mtk_dsi.c

@@ -598,19 +598,12 @@ static int mtk_dsi_poweron(struct mtk_dsi *dsi)
 	if (++dsi->refcount != 1)
 		return 0;
 
-	switch (dsi->format) {
-	case MIPI_DSI_FMT_RGB565:
-		bit_per_pixel = 16;
-		break;
-	case MIPI_DSI_FMT_RGB666_PACKED:
-		bit_per_pixel = 18;
-		break;
-	case MIPI_DSI_FMT_RGB666:
-	case MIPI_DSI_FMT_RGB888:
-	default:
-		bit_per_pixel = 24;
-		break;
+	ret = mipi_dsi_pixel_format_to_bpp(dsi->format);
+	if (ret < 0) {
+		dev_err(dev, "Unknown MIPI DSI format %d\n", dsi->format);
+		return ret;
 	}
+	bit_per_pixel = ret;
 
 	dsi->data_rate = DIV_ROUND_UP_ULL(dsi->vm.pixelclock * bit_per_pixel,
 					  dsi->lanes);
@@ -793,12 +786,11 @@ mtk_dsi_bridge_mode_valid(struct drm_bridge *bridge,
 			  const struct drm_display_mode *mode)
 {
 	struct mtk_dsi *dsi = bridge_to_dsi(bridge);
-	u32 bpp;
+	int bpp;
 
-	if (dsi->format == MIPI_DSI_FMT_RGB565)
-		bpp = 16;
-	else
-		bpp = 24;
+	bpp = mipi_dsi_pixel_format_to_bpp(dsi->format);
+	if (bpp < 0)
+		return MODE_ERROR;
 
 	if (mode->clock * bpp / dsi->lanes > 1500000)
 		return MODE_CLOCK_HIGH;