iommu/arm-smmu: Cleanup arm_smmu_init()

arm_smmu_init() has retained some workarounds for
back when it could be invoked multiple times as either
part of the initcall sequences, or through the IOMMU
OF table. This was necessary before the driver
core handled dependencies between an IOMMU master
and an IOMMU device. Since this is no longer the case,
we can remove this logic. Also, while we're here,
unregister the TBU driver if the SMMU driver cannot
be registered.

Change-Id: I5c2ed4a046c63babb3bd60390c468a4e5e27de6e
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>