drm/ast: fix mode_valid's return type

The method struct drm_connector_helper_funcs::mode_valid is defined
as returning an 'enum drm_mode_status' but the driver implementation
for this method uses an 'int' for it.

Fix this by using 'enum drm_mode_status' in the driver too.
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180424131443.1810-1-luc.vanoostenryck@gmail.com

drivers/gpu/drm/ast/ast_mode.c

@@ -799,7 +799,7 @@ static int ast_get_modes(struct drm_connector *connector)
 	return 0;
 }
 
-static int ast_mode_valid(struct drm_connector *connector,
+static enum drm_mode_status ast_mode_valid(struct drm_connector *connector,
 			  struct drm_display_mode *mode)
 {
 	struct ast_private *ast = connector->dev->dev_private;