iris: Drop 'engine' from iris_batch.

iris: Drop 'engine' from iris_batch.

For the moment, everything is I915_EXEC_RENDER, so this isn't necessary.
But even should that change, I don't think we want to handle multiple
engines in this manner.

Nowadays, we have batch->name (IRIS_BATCH_RENDER, IRIS_BATCH_COMPUTE,
possibly an IRIS_BATCH_BLIT for blorp batches someday), which describes
the functional usage of the batch.  We can simply check that and select
an engine for that class of work (assuming there ever is more than one).

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3613>