iris: Make an iris_foreach_batch macro that skips unsupported batches

iris: Make an iris_foreach_batch macro that skips unsupported batches

IRIS_BATCH_BLITTER isn't supported prior to Tigerlake; in general,
batches may not be supported on all hardware.  In most cases, querying
them is harmless (if useless): they reference nothing, have no commands
to flush, and so on.  However, the fence code does need to know that
certain batches don't exist, so it can avoid adding inter-batch fences
involving them.

This patch introduces a new iris_foreach_batch() iterator macro that
walks over all batches that are actually supported on the platform,
while skipping the others.  It provides a central place to update should
we add or reorder more batches in the future.

Fixes various tests in the piglit.spec.ext_external_objects.* category.

Thanks to Tapani Pälli for catching this.

Fixes: a90a1f15 ("iris: Create an IRIS_BATCH_BLITTER for using the BLT command streamer")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14834>