drm/xe: Export xe_step_name for kunit tests

drm/xe: Export xe_step_name for kunit tests

xe_step_name() is used by xe_assert(), so adding assertions to functions
like xe_device_get_gt() will result in

  ERROR: modpost: "xe_step_name" [drivers/gpu/drm/xe/tests/xe_test.ko] undefined!

while building the kunit tests.  Export xe_step_name to avoid these
build failures when adding assertions.

Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250701201320.2514369-11-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>