drm/mediatek: Support "None" blending in OVL

Support "None" alpha blending mode on MediaTek's chips.
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240717-alpha-blending-v4-1-4b1c806c0749@mediatek.com/Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_disp_ovl.c

@@ -434,6 +434,7 @@ void mtk_ovl_layer_config(struct device *dev, unsigned int idx,
 	unsigned int fmt = pending->format;
 	unsigned int offset = (pending->y << 16) | pending->x;
 	unsigned int src_size = (pending->height << 16) | pending->width;
+	unsigned int blend_mode = state->base.pixel_blend_mode;
 	unsigned int ignore_pixel_alpha = 0;
 	unsigned int con;
 	bool is_afbc = pending->modifier != DRM_FORMAT_MOD_LINEAR;
@@ -463,7 +464,8 @@ void mtk_ovl_layer_config(struct device *dev, unsigned int idx,
 	 * For RGB888 related formats, whether CONST_BLD is enabled or not won't
 	 * affect the result. Therefore we use !has_alpha as the condition.
 	 */
-	if (state->base.fb && !state->base.fb->format->has_alpha)
+	if ((state->base.fb && !state->base.fb->format->has_alpha) ||
+	    blend_mode == DRM_MODE_BLEND_PIXEL_NONE)
 		ignore_pixel_alpha = OVL_CONST_BLEND;
 
 	if (pending->rotation & DRM_MODE_REFLECT_Y) {