drm/sched: Add several job helpers to avoid drivers touching scheduler state

drm/sched: Add several job helpers to avoid drivers touching scheduler state

In the past, drivers used to reach into scheduler internals—this must
end because it makes it difficult to change scheduler internals, as
driver-side code must also be updated.

Add helpers to check if the scheduler is stopped and to query a job’s
signaled state to avoid reaching into scheduler internals. These are
expected to be used driver-side in recovery and debug flows.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Link: https://patch.msgid.link/20251209200039.1366764-2-matthew.brost@intel.com