intel-iommu: use coherent_dma_mask in alloc_coherent

This patch fixes intel-iommu to use dev->coherent_dma_mask in
alloc_coherent. Currently, intel-iommu uses dev->dma_mask in
alloc_coherent but alloc_coherent is supposed to use
coherent_dma_mask. It could break drivers that uses smaller
coherent_dma_mask than dma_mask (though the current code works for the
majority that use the same mask for coherent_dma_mask and dma_mask).

[dwmw2: dma_mask can be bigger than 'unsigned long']
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Reviewed-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>