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 <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

include/drm/drm_crtc_helper.h

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