drm/ttm: Fix configuration error around populate_and_map() functions

Fixed kbuild errors when IOMMU/SWIOTLB are disabled.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>