drm/ttm: Add a ttm_ref_object_exists function

A function to be used to check whether a caller has put a ref object
(opened) a struct ttm_base_object

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

include/drm/ttm/ttm_object.h

@@ -244,6 +244,10 @@ extern void ttm_base_object_unref(struct ttm_base_object **p_base);
 extern int ttm_ref_object_add(struct ttm_object_file *tfile,
 			      struct ttm_base_object *base,
 			      enum ttm_ref_type ref_type, bool *existed);
+
+extern bool ttm_ref_object_exists(struct ttm_object_file *tfile,
+				  struct ttm_base_object *base);
+
 /**
  * ttm_ref_object_base_unref
  *