i965: add MAYBE_UNUSED to assert param

This fixes an unused variable warning on release builds.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -5866,7 +5866,7 @@ fs_visitor::allocate_registers(bool allow_spilling)
    schedule_instructions(SCHEDULE_POST);
 
    if (last_scratch > 0) {
-      unsigned max_scratch_size = 2 * 1024 * 1024;
+      MAYBE_UNUSED unsigned max_scratch_size = 2 * 1024 * 1024;
 
       prog_data->total_scratch = brw_get_scratch_size(last_scratch);