mm: remove CONFIG_MIGRATE_VMA_HELPER

CONFIG_MIGRATE_VMA_HELPER guards helpers that are required for proper
devic private memory support.  Remove the option and just check for
CONFIG_DEVICE_PRIVATE instead.

Link: https://lore.kernel.org/r/20190814075928.23766-11-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/gpu/drm/nouveau/Kconfig

@@ -89,7 +89,6 @@ config DRM_NOUVEAU_SVM
 	depends on MMU
 	depends on STAGING
 	select HMM_MIRROR
-	select MIGRATE_VMA_HELPER
 	select MMU_NOTIFIER
 	default n
 	help

mm/Kconfig

@@ -669,9 +669,6 @@ config ZONE_DEVICE
 
 	  If FS_DAX is enabled, then say Y.
 
-config MIGRATE_VMA_HELPER
-	bool
-
 config DEV_PAGEMAP_OPS
 	bool
 

mm/migrate.c

@@ -2117,7 +2117,7 @@ out_unlock:
 
 #endif /* CONFIG_NUMA */
 
-#if defined(CONFIG_MIGRATE_VMA_HELPER)
+#ifdef CONFIG_DEVICE_PRIVATE
 static int migrate_vma_collect_hole(unsigned long start,
 				    unsigned long end,
 				    struct mm_walk *walk)
@@ -2942,4 +2942,4 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
 	}
 }
 EXPORT_SYMBOL(migrate_vma_finalize);
-#endif /* defined(MIGRATE_VMA_HELPER) */
+#endif /* CONFIG_DEVICE_PRIVATE */