drm/ast: Create threshold values for AST2600

The threshold value is used for AST2600 only.
Signed-off-by: KuoHsiang Chou <kuohsiang_chou@aspeedtech.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220117083643.41493-1-kuohsiang_chou@aspeedtech.com

drivers/gpu/drm/ast/ast_mode.c

@@ -471,7 +471,10 @@ static void ast_set_color_reg(struct ast_private *ast,
 static void ast_set_crtthd_reg(struct ast_private *ast)
 {
 	/* Set Threshold */
-	if (ast->chip == AST2300 || ast->chip == AST2400 ||
+	if (ast->chip == AST2600) {
+		ast_set_index_reg(ast, AST_IO_CRTC_PORT, 0xa7, 0xe0);
+		ast_set_index_reg(ast, AST_IO_CRTC_PORT, 0xa6, 0xa0);
+	} else if (ast->chip == AST2300 || ast->chip == AST2400 ||
 	    ast->chip == AST2500) {
 		ast_set_index_reg(ast, AST_IO_CRTC_PORT, 0xa7, 0x78);
 		ast_set_index_reg(ast, AST_IO_CRTC_PORT, 0xa6, 0x60);