drm/virtio: move byteorder handling into virtio_gpu_cmd_transfer_to_host_2d function

Be consistent with the rest of the code base.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191018122352.17019-1-kraxel@redhat.com