iommu: Factor out some helpers

The pattern for picking the first device out of the group list is
repeated a few times now, so it's clearly worth factoring out, which
also helps hide the iommu_group_dev detail from places that don't need
to know. Similarly, the safety check for dev_iommu_ops() at certain
public interfaces starts looking a bit repetitive, and might not be
completely obvious at first glance, so let's factor that out for clarity
as well, in preparation for more uses of both.

Reviewed-by: Lu Baolu <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Jerry Snitselaar <[email protected]>
Signed-off-by: Robin Murphy <[email protected]>
Link: https://lore.kernel.org/r/566cbd161546caa6aed49662c9b3e8f09dc9c3cf.1700589539.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <[email protected]>

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