mesa: correctly calculate the storage offset for i915
When generating the storage offset for struct members we need to skip opaque types as they no longer have backing storage. Fixes: fcbb93e8 ("mesa: stop assigning unused storage for non-bindless opaque types") V2: simplify since bindless will never be supported in this code Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101983 Reviewed-by:Kenneth Graunke <kenneth@whitecape.org>