msm: iommu: add stubs for convenience domain-finding functions

Recently, some convenience functions were added for working with domains
on MSM platforms [8bef2bfc: "msm: iommu: add some more convenience
domain-finding functions"]. However, on platforms where CONFIG_MSM_IOMMU
is not enabled those functions are never defined. Fix this by adding
stubs.

Change-Id: Ia70e3ceacc515d00ed8698a05907f5bc8db8cdc6
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>