nir/xfb: Fix offset accounting for dvec3/4

Before, we were double-counting the component slots when we had a dvec3
or dvec4.  Instead, just add them in once and manually offset the
recorded output offset.

Fixes: 19064b8c "nir: Add a pass for gathering transform feedback info"
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
(cherry picked from commit 4f99ac91)