drm: Preserve the list head in drm_mode_copy

Preserve the destination mode's list head in drm_mode_copy. Just
in case someone decides that it's a good idea to overwrite a mode which
happens to be on some list,

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_modes.c

@@ -789,16 +789,17 @@ EXPORT_SYMBOL(drm_mode_set_crtcinfo);
  * LOCKING:
  * None.
  *
- * Copy an existing mode into another mode, preserving the object id
- * of the destination mode.
+ * Copy an existing mode into another mode, preserving the object id and
+ * list head of the destination mode.
  */
 void drm_mode_copy(struct drm_display_mode *dst, const struct drm_display_mode *src)
 {
 	int id = dst->base.id;
+	struct list_head head = dst->head;
 
 	*dst = *src;
 	dst->base.id = id;
-	INIT_LIST_HEAD(&dst->head);
+	dst->head = head;
 }
 EXPORT_SYMBOL(drm_mode_copy);