drm/xe/mocs: Clarify difference between hw and sw sizes

It's not very obvious what the difference is between the 'size' and
'n_entries' fields of the MOCS structure.  Rename both fields slightly
and add some comments explaining that one is the documentation-defined
table size, while the other is the number of entries that can be
programmed into the hardware (and the documented table size can
potentially be smaller than the number of hardware entries).

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/20240627203741.2042752-4-matthew.d.roper@intel.com