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.