drm/xe: Fix PTE index in xe_vm_populate_pgtable() for chunked binds

drm/xe: Fix PTE index in xe_vm_populate_pgtable() for chunked binds

xe_vm_populate_pgtable() indexed the source PTE array (update->pt_entries)
by the per-call loop counter, assuming each call starts at the first entry
of the update. That holds for the CPU bind path
(xe_migrate_update_pgtables_cpu), which populates a whole update in a single
call, but not for the GPU bind path: write_pgtable() splits an update into
MAX_PTE_PER_SDI (510) sized MI_STORE_DATA_IMM chunks, invoking the populate
callback once per chunk with an advancing qword_ofs but a fresh command-
buffer destination pointer.

As a result, every chunk after the first re-read pt_entries from index 0
instead of from its true offset, so PTEs beyond the first 510 entries of a
single update were programmed with the wrong physical pages, shifting the
mapping by exactly MAX_PTE_PER_SDI pages.

This stayed latent because a single update only exceeds 510 qwords when a
large (e.g. 2M) region is bound as individual 4K PTEs rather than a single
huge-page entry, which happens when the backing store is sufficiently
fragmented. It was surfaced by the BO defrag path, which deliberately
rebinds such fragmented ranges via the GPU bind path, producing
deterministic data corruption offset by 510 pages.

Index pt_entries by the chunk's absolute offset relative to update->ofs so
both the CPU and GPU paths pick the correct entries.

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: stable@vger.kernel.org
Assisted-by: GitHub_Copilot:claude-opus-4.8
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patch.msgid.link/20260702012434.3861171-1-matthew.brost@intel.com
(cherry picked from commit e6f2d0b757c4fb577a513c577140109d1d292a9a)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>