drm/xe/kunit: Kill xe_cur_kunit()

We shouldn't use custom helper if there is a official one.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240705191057.1110-2-michal.wajdeczko@intel.com