The state tracker generates several built-in shaders in order to
perform scissored clears, upload/download PBOs, and so on.  These
are currently constructed using TGSI, using ureg and u_simple_shader.

I want to have NIR versions of these shaders, for my Gallium driver
that has a NIR backend but no TGSI support.  To that end, we'll want
a few helpers to help construct simple shaders.

This patch adds two new helpers:

- st_nir_finish_builtin_shader() takes a manually constructed NIR
  shader, applies lowering passes (like st_link_nir would do for GLSL),
  and constructs the pipe_shader_state.

- st_nir_make_passthrough_shader() makes a simple passthrough shader,
  which copies inputs to outputs.  This is similar to u_simple_shaders.

v2: Set info->fs.untyped_color_outputs for vc4/v3d (thanks Eric!).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Tested-by: Eric Anholt <eric@anholt.net>