drm/xe: Fix xe_pt_abort_unbind

When restoring the children PT entries on a bind failure the incorrect
loop index has used resulting in PT entries being leaked. This is shown
by running xe_vm.bind-array-conflict-error-inject on a VRAM device going
into a suspend state after the test completes.

v2:
 - s/childern/children (CI, Matt Auld)

Fixes: a708f650 ("drm/xe: Update PT layer with better error handling")
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240723010230.1652707-1-matthew.brost@intel.com