Change pre move of Pmac Trajectory to use direct demands

[tomtrafford]

          In a separate issue we should do this via the direct demands deferred move like https://github.com/dls-controls/pymalcolm/blob/2c3d7048026ef9c5a0fcd69db89bd88d74f62ad9/malcolm/modules/pmac/parts/cspart.py#L53-L85

Originally posted by @coretl in #440 (comment)