Whatever functionalities PDCP layer supports are not needed by RLC TM mode data. TM Data is associated with one of the logical channels (CCCH/BCCH/PCCH). Common control channel is common for all the UEs so no need of security for this channel. BCCH channel carries broadcasting information which is also common for all the UEs. PCCH is also a channel which is not per UE, paging channel is used to page set of UEs which are in idle mode.
Due to these above reasons, PDCP layer does not comes between from upper layer and RLC TM data.