PRSM is required, as it is the only UI for the AVC and WSE features. ASDM will only allow you to redirect traffic to the CS module (service policy setup). Applying policy to the redirected traffic (and any monitoring / reporting) is all done via the PRSM user interface.
However you can use the single device (on-box) version that is included with the image on the CX module. Multi-device PRSM ("off-box") is optional and only really needed when managing multiple ASA CX modules from a single place (though it it is useful for even a small HA pair as you need to update each member of the HA cluster separately otherwise).