drm/xe/mocs: Move warn/assertion up

The warn-once in __init_mocs_table() to make sure there's an index set
for unused entries is more a sanity check that should be done as the
first thing in that function. The kunit test replicates the same check,
so also move it up and turn it into a failure condition for the test.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240228061048.3661978-4-lucas.demarchi@intel.com


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