System may be able to get physical address of zero if not reserved by
firmware.

The dma address obtained by 'dma_alloc_coherent' is valid, since already
checking cpu va before, so do not check again.
Signed-off-by: Hui Tang <tanghui20@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>