mesa/st: move cloning of NIR shader for compute

mesa/st: move cloning of NIR shader for compute

Since in the NIR case, driver takes ownership of the NIR shader, we need
to clone what is passed to the driver.  Normally this is done as part of
creating the shader variant (where is clone is anyways needed).  But
compute shaders have no variants, so we were cloning earlier.

The problem is that after the NIR linking optimizations, we ended up
cloning *before* all the lowering passes where done.

So move this into st_get_cp_variant(), to make compute shaders work more
like other shader stages.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>