drm/xe/mocs: Drop xe_mocs_info_index

The values in the xe_mocs_info_index enum only match old pre-gen12
hardware not supported by the Xe driver.

The only usage of this enum was to set a default value for
info->unused_entries_index, but this is unnecessary since every platform
in the subsequent switch statement sets a proper platform-specific value
(and the XE_MOCS_PTE default doesn't even make sense since the hardware
dropped the "use PAT settings" capability in gen12).

v2:
 - Add a check that unusued_entries_index is non-zero; even for
   platforms where this is a valid table entry, it's never the one we
   want this value assigned to.  (Lucas)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>