iris: Fix large timeout handling in rel2abs()

...by copying the implementation of anv_get_absolute_timeout().

Appears to fix a CTS test with 32-bit builds:
GTF-GL46.gtf32.GL3Tests.sync.sync_functionality_clientwaitsync_flush

Fixes: f459c56b ("iris: Add fence support using drm_syncobj")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
(cherry picked from commit 7ee7b0ec)