powerpc/powernv: Fix iommu initialization again

So because those things always end up in trainwrecks... In 7846de40
we moved back the iommu initialization earlier, essentially undoing
37f02195 which was causing us endless trouble... except that in the
meantime we had merged 959c9bdd (to workaround the original breakage)
which is now ... broken :-)

This fixes it by doing a partial revert of the latter (we keep the
ppc_md. path which will be needed in the hotplug case, which happens
also during some EEH error recovery situations).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@vger.kernel.org> [v3.10]