zink: implement nir_texop_txd

zink: implement nir_texop_txd

This lets us enable PIPE_CAP_FRAGMENT_SHADER_TEXTURE_LOD, which in turns
gives us ARB_shader_texture_lod.

Still fails one piglit test on ANV, namely
spec@arb_shader_texture_lod@execution@arb_shader_texture_lod-texgradcube,
but with 33 new passing tests, I think this is worth it.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3140>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3140>