drm/xe: Combine destroy_cb and destroy_work in xe_vma into union

The callback kicks the worker thus mutually exclusive execution,
combining saves a bit of space in xe_vma.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_vm_types.h

@@ -61,11 +61,12 @@ struct xe_vma {
 		struct list_head destroy;
 	} combined_links;
 
+	union {
 		/** @destroy_cb: callback to destroy VMA when unbind job is done */
 		struct dma_fence_cb destroy_cb;
-
 		/** @destroy_work: worker to destroy this BO */
 		struct work_struct destroy_work;
+	};
 
 	/** @userptr: user pointer state */
 	struct {