drm/ttm: Avoid NULL pointer deref for evicted BOs

In the Linux kernel, the following vulnerability has been resolved: drm/ttm: Avoid NULL pointer deref for evicted BOs It is possible for a BO to exist that is not currently associated with a resource, e.g. because it has been evicted. When devcoredump tries to read the contents of all BOs for dumping, we need to expect this as well -- in this case, ENODATA is recorded instead of the buffer contents.