pldm_visualise_pdrs.py: use a generator
pldm_visualise_pdrs.py: use a generator

Factor the logic and implementation of iterating over PDRs out from the
logic of sorting data into the various pdr type containers in
fetch_pdrs_from_bmc.  Move the factored-out logic to a new generator
method get_pdrs.  This facilitates new ways of getting PDRs into the
program without disturbing the code that processes them.

Change-Id: I23e9b56b3ec1d275f93fef51edeed11d6de696bd
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/pldm,openbmc/pldm,openbmc/pldm