iris: don't mark contains_draw = false when chaining batches

iris: don't mark contains_draw = false when chaining batches

chaining to a new batch reuses create_batch(), but we don't need to do
the work of pinning BOs we inherit from a previous batch...when that is
actually part of the same execbuf invocation.

instead, just flag it when setting primary_batch_size = 0, in
iris_batch_reset