drm/xe: Stop abusing DRM scheduler internals

drm/xe: Stop abusing DRM scheduler internals

Use new pending job list iterator and new helper functions in Xe to
avoid reaching into DRM scheduler internals.

Part of this change involves removing pending jobs debug information
from debugfs and devcoredump. As agreed, the pending job list should
only be accessed when the scheduler is stopped. However, it's not
straightforward to determine whether the scheduler is stopped from the
shared debugfs/devcoredump code path. Additionally, the pending job list
provides little useful information, as pending jobs can be inferred from
seqnos and ring head/tail positions. Therefore, this debug information
is being removed.

v4:
 - Add comment around DRM_GPU_SCHED_STAT_NO_HANG (Niranjana)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Link: https://patch.msgid.link/20260110012739.2888434-3-matthew.brost@intel.com