drm/xe/kunit: Extend platform generator with PTL

Our list of typical platforms used to generate test device objects
does not contain any PANTHERLAKE. Add one.

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/20250818192032.633-1-michal.wajdeczko@intel.com