drm/xe/kunit: Update struct xe_pci_fake_data step declarations

drm/xe/kunit: Update struct xe_pci_fake_data step declarations

The struct xe_pci_fake_data has fields that specify graphics and
media stepping of the fake PCI device used during KUnit testing.

Change definitions of those separate step fields and use existing
struct xe_step_info definition that already have required fields.

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/20250829171922.572-3-michal.wajdeczko@intel.com