drm/xe: Guard teardown paths against purged BOs

VRAM page offlining can purge BOs that are still referenced by page
tables, exec queues, and DMA-buf exports. Add xe_bo_is_purged()
guards in the teardown paths to prevent unpinning or mapping an
already-purged BO:

- xe_bo_unpin_map_no_vm(): skip unpin if purged
- xe_dma_buf_map(): return -ENOENT early if purged
- xe_exec_queue_update_run_ticks(): skip LRC timestamp read if purged
- xe_pt_destroy(): skip unpin if purged

v4(Sashiko):
- Move bo purge check in xe_bo_unpin() to handle all instances
v3(Sashiko):
- Remove dma_resv is already held
v2(Himal):
- take dma_resv lock before calling xe_bo_is_purged()

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/20260903161553.528932-23-tejas.upadhyay@intel.com

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