glsl: don't lower variable indexing on non-patch tessellation inputs/outputs

glsl: don't lower variable indexing on non-patch tessellation inputs/outputs

There is no way to lower them, because the array sizes are unknown
at compile time.

Based on a patch from: Fabian Bieler <fabianbieler@fastmail.fm>

v2: add comments

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>