drirc: setup anv_assume_full_subgroups=16 for UnrealEngine5.1

We set max subgroup size as 16 for 'UnrealEngine5.1', this improves a
customer benchmark by 50% on A750.

Cc: mesa-stable
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26385>

src/util/00-mesa-defaults.conf

@@ -1202,6 +1202,7 @@ TODO: document the other workarounds.
         <!-- Disable FCV optimization for Unreal Engine 5.1 workloads. -->
         <engine engine_name_match="UnrealEngine5.1">
             <option name="anv_disable_fcv" value="true" />
+            <option name="anv_assume_full_subgroups" value="16" />
         </engine>
     </device>
     <device driver="dzn">