drm/xe: Limit number of jobs per exec queue

Add a limit to the number of jobs that can be queued in a single
exec queue to avoid potential resource exhaustion.

A new field `job_cnt` is introduced in `struct xe_exec_queue` to
track the number of active DRM jobs, along with a maximum limit
`XE_MAX_JOB_COUNT_PER_EXEC_QUEUE` set to 1000.

If the job count exceeds this threshold, `xe_exec_ioctl()` now
returns `-EAGAIN` to signal that the caller should retry later.

A trace event is added to track when the limit is reached:
"xe_exec_queue_reach_max_job_count: dev=0000:03:00.0, job count
exceeded the maximum limit (1000) per exec queue. engine_class=0x3,
logical_mask=0x1, guc_id=2"

v3: add assert in xe_exec_queue_destroy that q->job_cnt is zero. (Matt)
v2 (Matt):
 - add log to trace the limit is hit.
 - Change max count from 0x1000 to 1000.
 - Use atomic_t for job_cnt.

Suggested-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251027202118.3339905-2-shuicheng.lin@intel.com

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