iommu: Fix compilation errors without CONFIG_IOMMU_API

Move code into appropriate #ifdef, and add an "inline" so the compiler no
longer gives a defined-but-not-used warning.

Change-Id: I226242bf994e659d8018a885f20c3a3f80b9b8e5
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>