spapr: Rework DRC name handling

spapr: Rework DRC name handling

DRC objects have a get_name method which returns the DRC name generated
when the DRC is created.  Replace that with a fixed spapr_drc_name()
function which generates the name on the fly from other information.  This
means:
  * We get rid of a method with only one implementation, and only local
    callers
  * We don't have to carry the name string around for the lifetime of the
    DRC
  * We use information added to the class structure to generate the name
    in standard format, so we don't need an explicit switch on drc type
    any more

We also eliminate the 'name' property; it's basically useless since the
only information in it can easily be deduced from other things.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Acked-by: Michael Roth <mdroth@linux.vnet.ibm.com>