iris: Track written BOs via a bitfield rather than exec_object2 entries

iris: Track written BOs via a bitfield rather than exec_object2 entries

When we start suballocating BOs, multiple logical BOs may map to the
same GEM object, and thus share a validation list entry.  However, we
want to track whether logical BOs are written, to avoid unnecessary
cross-batch data dependencies.

Just track that in a bitfield instead, where bit i corresponds to
batch->exec_bos[i].

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12848>