iris: remove hole from struct iris_bo

iris: remove hole from struct iris_bo

This decreases the size of the struct on a 64bit machine from 144 to
136. While that's not a lot, this is one of the structs that we're
allocating all the time.

For a full Aztec run on BDW we allocate this struct 3273 times, and we
can have up to 3259 of them live at the same time. So we end up saving
just a little over 6 pages for this benchmark.

Spotted this while trying to add another bool for an unrelated
feature.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4561>