drm/xe: Fix xe_exec_queue_is_idle for parallel exec queues

Last little piece to support parallel exec queue is compute mode.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>