drm/xe/mocs: Fix DG2 kunit

LNCFCMOCS31[31:16] is read-only for DG2 and MTL, so it's not possible to
check set it. While trying to set doesn't cause any issue, later when
it's read back to check if the value got correctly recorded causes the
test to fail. Now that test is reliable for an odd number of entries,
reduce it so the last entry is ignored.

Bspec: 55267
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1253
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1233


Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240228061048.3661978-6-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>