The behaviour if you enable PASID support after ATS is undefined. So we
have to enable it first, even if we don't know whether we'll need it.

This is safe enough; unless we set up a context that permits it, the device
can't actually *do* anything with it.

Also shift the feature detction to dmar_insert_one_dev_info() as it only
needs to happen once.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>