drm/xe: Link LRC BO and its execution queue with safe lifetime rules

Introduce an execution queue back-pointer (`q`) within `struct xe_bo`, primarily
for Logical Ring Context (LRC) Buffer Objects. This back-pointer allows the
driver to identify and execute targeted corrective actions on a specific queue
if its associated LRC BO encounters errors like memory corruption or eviction.

Because this back-pointer takes no reference on its target execution queue, strict
lifetime and serialization rules are implemented to prevent concurrent readers
from encountering use-after-free or dangling pointer bugs:

- Encapsulate tracking logic inside xe_exec_queue_set_lrc_bo_backpointer() and
  xe_exec_queue_clear_lrc_bo_backpointer().

- Explicitly wrap all back-pointer writes and clears under the BO's dma_resv lock
  via xe_bo_lock(). Readers must hold this same lock across both the pointer read
  and its subsequent xe_exec_queue_get_unless_zero() call to guarantee serialization
  against teardown.

- Defer publishing the back-pointer until the very end of xe_exec_queue_create().
  This ensures that early initialization failure paths (which bypass the kref
  mechanism and immediately free the queue structure) never leak a transient pointer
  to concurrent readers.

- Clear the back-pointer at the absolute top of __xe_exec_queue_fini(). This strips
  the pointer before q->ops->fini() destroys the hardware backend, ensuring that
  any reader holding the BO lock either observes a fully functional queue or NULL.

For multi-queue engines, secondary LRC BOs safely point to the primary queue, which
is guaranteed to outlive the teardown pass due to active references held by its
secondaries.

V4 (MattB):
  - Add LRC BO's execution queue safe lifetime rules
V3 (Sashiko):
  - Use 8-byte placeholder structure compatibility for non-LRC BO cases.
  - Wrap assignments and clears securely under dma_resv locks.
V2 (Matt B):
  - Add native support handling multi-queue configuration tracking.

Assisted-by: Copilot:claude-opus-4.6
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260903161553.528932-19-tejas.upadhyay@intel.com

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