v2 (MattR):
 - Clarify comment above RING_CMD_CCTL programming.
 - Remove bspec reference from field definition.  (Lucas)
 - Add WARN if we try to use a (presumably uninitialized) wb_index of 0.
   On most platforms 0 is an invalid MOCS entry and even on the ones
   where it isn't, it isn't the right setting for wb_index.  (Lucas)

Bspec: 45101, 72161
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ayaz A Siddiqui <ayaz.siddiqui@intel.com>
Signed-off-by: Fei Yang <fei.yang@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220505213812.3979301-4-matthew.d.roper@intel.com