drm/xe/kunit: Promote fake platform parameter list

The list of all known representative platforms defined in xe_wa
could be used in more places by other test suites.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250829171922.572-6-michal.wajdeczko@intel.com