nir: Fix deref offset calculation for structs.

We were calcuating the offset for the field within the struct, and just
dropping it on the floor.  Fixes a regression in
KHR-GLES3.shaders.struct.local.nested_struct_array_dynamic_index_fragment
and a few of its friends since the scratch lowering commit.

Fixes: e8e159e9 ("nir/deref: Add helpers for getting offsets")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 9ac5ec2f)