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 <1d31d94f30d40df7951505d1034e1e923d02ec49@basnieuwenhuizen.nl>
Reviewed-by: Edward O'Callaghan <b035da0d7e151e09fe2005f3a7bc526beb9b3245@folklore1984.net>
Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler