iris: make BATCH_SZ smaller by BATCH_RESERVED bytes

iris: make BATCH_SZ smaller by BATCH_RESERVED bytes

Iris allocates gem buffers using buckets of allocation sizes that are
page aligned. We always ask for batch buffers of size BATCH_SZ +
BATCH_RESERVED, which is not page aligned: we ask for 65552 bytes,
which ends up in the bucket of size 81920, resulting in 20% unused
space. Adjust things so there is no waste of space: BATCH_SZ +
BATCH_RESERVED is now 65536.

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>