There's nothing in NIR which guarantees that the deref is the first
source or that the coordinate is the second. Use
nir_tex_instr_src_index() to get the actual indices.
Fixes: 84b08971fb ("nir/lower_input_attachments: lower nir_texop_fragment_{mask}_fetch")
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35551>