drm/xe: fix error handling in xe_migrate_update_pgtables

Don't call drm_suballoc_free with sa_bo pointing to PTR_ERR.

References: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/2120
Fixes: dd08ebf6

 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
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: <stable@vger.kernel.org> # v6.8+
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240620102025.127699-2-matthew.auld@intel.com