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_drv.h b/drivers/gpu/drm/ast/ast_drv.h
index fab4b17..5617427 100644
--- a/drivers/gpu/drm/ast/ast_drv.h
+++ b/drivers/gpu/drm/ast/ast_drv.h
@@ -61,6 +61,7 @@
 	AST2200,
 	AST2150,
 	AST2300,
+	AST2400,
 	AST1180,
 };