arm64: dma-mapping: Clean up arm_iommu_attach_device() call flow

Previously the code assumed that iommu_attach_group() would occur after
setting up any dma-specific data structures. But there are several places
where to satisfy this requirement we attach, check some iommu attribute,
and then detach. This is not very clean, so improve it.

Change-Id: I5f115217c892716e3f3911b8177f530a64873267
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>