mesa/st/nir: fix missing nir_compact_varyings

LinkedTransformFeedback is normally populated, which had nerf'd varying
packing since the check was introduced.

Fixes: dbd52585 st/nir: Disable varying packing when doing transform feedback.
Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit cfe82209)