dma-contiguous: fix comment for dma_release_from_contiguous

Commit 90ae409f ("dma-direct: fix zone selection
after an unaddressable CMA allocation") changed the logic in
dma_release_from_contiguous to remove the normal pages fallback path,
but did not update the comment. Fix that.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>