dma-mapping-fast: Reorganize common behavior

fast_smmu_mmap_attrs(), fast_smmu_get_sgtable(), and fast_smmu_free()
share the need to classify an input address into one of several cases.
Write them in the same general format.

Change-Id: I237f76151a47491ba9ecf7c1701eed49de20cd7e
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>