vdpa/mlx5: Use the correct dma device when registering memory

In cases where the vdpa instance uses a SF (sub function), the DMA
device is the parent device. Use a function to retrieve the correct DMA
device.

Fixes: 1958fc2f

 ("net/mlx5: SF, Add auxiliary device driver")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Link: https://lore.kernel.org/r/20210408091047.4269-3-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>