i965/nir: enable lowering of texture gradient for cube maps

This gets the lowering on the Vulkan driver too.

Fixes Vulkan CTS cube map texture gradient tests in:
dEQP-VK.glsl.texture_functions.texturegrad.*

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_nir.c

@@ -467,6 +467,7 @@ brw_preprocess_nir(const struct brw_compiler *compiler, nir_shader *nir)
       .lower_txp = ~0,
       .lower_txf_offset = true,
       .lower_rect_offset = true,
+      .lower_txd_cube_map = true,
    };
 
    OPT(nir_lower_tex, &tex_options);