iris: Drop redundant iris_address::write flag.

The write flag is redundant since it can be inferred easily from the
iris_address::access domain.  This allows the iris_address struct to
be laid out more efficiently in memory, leading to a measurable
improvement in several Piglit Drawoverhead test-cases.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3875>
3 files changed