iris: fix SSBO indexing

iris: fix SSBO indexing

st/nir offsets SSBO indexes by MaxABOs.  This is not what we want,
as it bloats the binding tables.  We'll need to adjust it to use
info->num_abos as the offset and buffer base instead.  For now,
just use the inefficient format to get us rolling.  We can add a
PIPE_CAP later.