drm/ast: add AST 2400 support.

This is ported from the userspace driver.

Untested on any ast2400 hw so far.

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
index 72bec23..e9a14a1 100644
--- a/drivers/gpu/drm/ast/ast_mode.c
+++ b/drivers/gpu/drm/ast/ast_mode.c
@@ -398,7 +398,7 @@
 	ast_set_index_reg_mask(ast, AST_IO_CRTC_PORT, 0xa8, 0xfd, jregA8);
 
 	/* Set Threshold */
-	if (ast->chip == AST2300) {
+	if (ast->chip == AST2300 || ast->chip == AST2400) {
 		ast_set_index_reg(ast, AST_IO_CRTC_PORT, 0xa7, 0x78);
 		ast_set_index_reg(ast, AST_IO_CRTC_PORT, 0xa6, 0x60);
 	} else if (ast->chip == AST2100 ||
@@ -771,7 +771,7 @@
 		if ((mode->hdisplay == 1600) && (mode->vdisplay == 900))
 			return MODE_OK;
 
-		if ((ast->chip == AST2100) || (ast->chip == AST2200) || (ast->chip == AST2300) || (ast->chip == AST1180)) {
+		if ((ast->chip == AST2100) || (ast->chip == AST2200) || (ast->chip == AST2300) || (ast->chip == AST2400) || (ast->chip == AST1180)) {
 			if ((mode->hdisplay == 1920) && (mode->vdisplay == 1080))
 				return MODE_OK;