drm/xe/svm: Document how xe keeps drm_pagemap references

drm/xe/svm: Document how xe keeps drm_pagemap references

As an aid to understanding the lifetime of the drm_pagemaps used
by the xe driver, document how the xe driver keeps the
drm_pagemap references.

v3:
- Fix formatting (Matt Brost)

Suggested-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251219113320.183860-19-thomas.hellstrom@linux.intel.com