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: 2e5bbb5461f1 ("KVM: PPC: Book3S HV: Migrate pinned pages out of CMA")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/mmu_context_iommu.c

@@ -184,7 +184,7 @@ long mm_iommu_get(struct mm_struct *mm, unsigned long ua, unsigned long entries,
 		 * of the CMA zone if possible. NOTE: faulting in + migration
 		 * can be expensive. Batching can be considered later
 		 */
-		if (get_pageblock_migratetype(page) == MIGRATE_CMA) {
+		if (is_migrate_cma_page(page)) {
 			if (mm_iommu_move_page_from_cma(page))
 				goto populate;
 			if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),