drm/xe/vm: fix xe_vm_validation_exec() kernel-doc

drm/xe/vm: fix xe_vm_validation_exec() kernel-doc

Fix kernel-doc warnings on xe_vm_validation_exec():

Warning: ../drivers/gpu/drm/xe/xe_vm.h:392 expecting prototype for
  xe_vm_set_validation_exec(). Prototype was for xe_vm_validation_exec()
  instead

Fixes: 0131514f9789 ("drm/xe: Pass down drm_exec context to validation")
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patch.msgid.link/20260107155401.2379127-4-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>