vdp/mlx5: Fix setting the correct dma_device

Before SF support was introduced, the DMA device was equal to
mdev->device which was in essence equal to pdev->dev.

With SF introduction this is no longer true. It has already been
handled for vhost_vdpa since the reference to the dma device can from
within mlx5_vdpa. With virtio_vdpa this broke. To fix this we set the
real dma device when initializing the device.

In addition, for the sake of consistency, previous references in the
code to the dma device are changed to vdev->dma_dev.

Fixes: d13a15d5

 ("vdpa/mlx5: Use the correct dma device when registering memory")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20210606053150.170489-1-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>