drm: Make the connector mode_valid() vfunc return a drm_mode_status enum

To make it clear what exactly mode_valid() should return.
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_crtc_helper.h

@@ -120,7 +120,7 @@ struct drm_encoder_helper_funcs {
  */
 struct drm_connector_helper_funcs {
 	int (*get_modes)(struct drm_connector *connector);
-	int (*mode_valid)(struct drm_connector *connector,
+	enum drm_mode_status (*mode_valid)(struct drm_connector *connector,
 					   struct drm_display_mode *mode);
 	struct drm_encoder *(*best_encoder)(struct drm_connector *connector);
 };