drm/amdgpu: make sure userptr ttm is allocated

We need to allocate that manually now.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Tested-by: Michel Dänzer <mdaenzer@redhat.com>
Link: https://patchwork.freedesktop.org/patch/384330/