drm/ttm: use gem vma_node

Drop vma_node from ttm_buffer_object, use the gem struct
(base.vma_node) instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190805140119.7337-9-kraxel@redhat.com