drm_global.c: Destroy sx in drm_global_release()

This fixes a build error at the same time (unused variable "item"), if
the kernel is compiled without INVARIANTS.

Reported by:	Hartmann, O. <ohartman@zedat.fu-berlin.de> (build error)
Reviewed by:	Konstantin Belousov (kib@)

sys/dev/drm2/drm_global.c

@@ -63,6 +63,7 @@ void drm_global_release(void)
 		struct drm_global_item *item = &glob[i];
 		MPASS(item->object == NULL);
 		MPASS(item->refcount == 0);
+		sx_destroy(&item->mutex);
 	}
 }