powerpc/mm: Build fix for non SPARSEMEM_VMEMAP config

We can use pfn_to_page() in realmode for other configs. Hence remove the
CONFIG_FLATMEM ifdef.

Fixes: 8e0861fa ("powerpc: Prepare to support kernel handling of IOMMU map/unmap")
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
[mpe: Also fix up the #endif comment]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>