Separate nFAPI PDCCH allocation from nFAPI DCI allocation

According to SCF222, a single PDCCH allocation groups DCIs that are
within the same BWP and CORESET. Therefore, if we want to allocate
multiple DCIs, we need to decouple PDCCH allocation and DCI (previously
jointly done in nr_configure_pdcch()), especially to be forward
compatible.

***Note that as of this commit, we would still allocate different PDCCH
PDUs for multiple UEs (which we do not support yet, anyway)***

nr_configure_pdcch(): simply take out DCI allocation.

nr_generate_Msg2(): separately allocate dci_pdu in common RA SS, and
rename DCI payload variable. Also, reorganize the function so that it is
first checked for CCE allocation and messages nFAPI messages are
allocated afterwards.

nr_schedule_ue_spec(): separately allocate dci_pdu in UE-specific SS.
Rename DCI payload variable.

nr_schedule_ulsch(): separately allocate dci_pdu in UE-specific SS.
Rename DCI payload variable.

nr_fill_nfapi_dl_sib1_pdu(): separately allocate dci_pdu in common SS.