nouveau: remove a few function stubs

nouveau_dmem_migrate_vma and nouveau_dmem_convert_pfn are only called
when CONFIG_DRM_NOUVEAU_SVM is enabled, so there is no need to provide
!CONFIG_DRM_NOUVEAU_SVM stubs for them.

Link: https://lore.kernel.org/r/20190814075928.23766-6-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>