powerpc/mm: Fix build break when CMA=n && SPAPR_TCE_IOMMU=y

powerpc/mm: Fix build break when CMA=n && SPAPR_TCE_IOMMU=y

Currently the build breaks if CMA=n and SPAPR_TCE_IOMMU=y:

  arch/powerpc/mm/mmu_context_iommu.c: In function ‘mm_iommu_get’:
  arch/powerpc/mm/mmu_context_iommu.c:193:42: error: ‘MIGRATE_CMA’ undeclared (first use in this function)
  if (get_pageblock_migratetype(page) == MIGRATE_CMA) {
  ^~~~~~~~~~~

Fix it by using the existing is_migrate_cma_page(), which evaulates to
false when CMA=n.

Fixes: 5d557c34362d ("KVM: PPC: Book3S HV: Migrate pinned pages out of CMA")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>