st/mesa: Add NIR versions of the clear shaders.

st/mesa: Add NIR versions of the clear shaders.

We implement the basic VS and FS, as well as the VS that does layered
clears by writing gl_Layer from the vertex shader.  Drivers which need
a geometry shader for writing layer continue falling back to TGSI, as
I didn't need this and so didn't bother implementing it.  (We certainly
could, however, if people want to add it in the future.)

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