drm/nouveau/vm: reduce number of entry-points to vm_map()

Pretty much everywhere had to make the decision which to use, so it
makes a lot more sense to just have one entrypoint decide the path
to take instead.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_ttm.c

@@ -171,6 +171,7 @@ nouveau_gart_manager_new(struct ttm_mem_type_manager *man,
 	node = kzalloc(sizeof(*node), GFP_KERNEL);
 	if (!node)
 		return -ENOMEM;
+
 	node->page_shift = 12;
 
 	switch (nv_device(drm->device)->card_type) {