drm/xe/pt: prevent invalid cursor access for purged BOs

drm/xe/pt: prevent invalid cursor access for purged BOs

During a page table walk for binding, xe_pt_stage_bind() explicitly
skips initializing the xe_res_cursor for purged BOs, treating them
similarly to NULL VMAs by only setting the cursor size.

However, xe_pt_hugepte_possible() and xe_pt_scan_64K() did not check
if the BO was purged before attempting to walk the cursor using
xe_res_dma() and xe_res_next(). Because the cursor was left
uninitialized for purged BOs, this falls through and triggers
warnings like:

  WARNING: drivers/gpu/drm/xe/xe_res_cursor.h:274 at xe_res_next

Fix this by explicitly checking if the BO is purged in both
xe_pt_hugepte_possible() and xe_pt_scan_64K(), returning early just
as we do for NULL VMAs, avoiding the invalid cursor accesses entirely.

As a precaution, also zero-initialize the cursor in xe_pt_stage_bind()
to ensure we don't pass garbage data into the page table walkers
if we ever hit a similar edge case in the future.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/8418
Fixes: ad9843aac91a ("drm/xe/madvise: Implement purgeable buffer object support")
Assisted-by: Copilot:gemini-3.1-pro-preview
Reported-by: Matthew Schwartz <matthew.schwartz@linux.dev>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Arvind Yadav <arvind.yadav@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Tested-by: Matthew Schwartz <matthew.schwartz@linux.dev>
Link: https://patch.msgid.link/20260625152054.450125-8-matthew.auld@intel.com
(cherry picked from commit 4c7b9c6ece32440e5a435a92076d049450cd2d2e)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>