radv/ac: add const_index to fetch index for gs inputs

radv/ac: add const_index to fetch index for gs inputs

This fixes clip distance fetches as they are single item loads
with a const_index like float[1].

Fixes:
dEQP-VK.clipping.user_defined.*.vert_geom.[0-6]

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>