iris: Support multiple chained batches.
There was never much point in artificially limiting chaining to two batches - we can trivially support arbitrary length chains. Currently, we should only ever have 1 or 2, but this may change. Reviewed-by:Tapani Pälli <tapani.palli@intel.com> Tested-by: Marge Bot <mesa/mesa!3613> Part-of: <mesa/mesa!3613>