sparc64/pci_sun4v: fix ATU checks for large DMA masks

Now that we allow drivers to always need to set larger than required
DMA masks we need to be a little more careful in the sun4v PCI iommu
driver to chose when to select the ATU support - a larger DMA mask
can be set even when the platform does not support ATU, so we always
have to check if it is avaiable before using it.  Add a little helper
for that and use it in all the places where we make ATU usage decisions
based on the DMA mask.

Fixes: 24132a41 ("sparc64/pci_sun4v: allow large DMA masks")
Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Meelis Roos <mroos@linux.ee>
Acked-by: David S. Miller <davem@davemloft.net>