nouveau/class_parser: Make strided element functions const

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33771>

src/nouveau/headers/struct_parser.py

@@ -25,7 +25,7 @@ use std::ops::Range;
     % for f in s.fields:
         % if f.stride:
 #[inline]
-pub fn ${s.name}_${f.name}(i: usize) -> Range<usize> {
+pub const fn ${s.name}_${f.name}(i: usize) -> Range<usize> {
     (i * ${f.stride} + ${f.lo})..(i * ${f.stride} + ${f.hi + 1})
 }
         % else: