st/program: fix compute shader nir references

st/program: fix compute shader nir references

In case the IR is NIR, the driver takes reference to the nir_shader.
Also, because there are no variants, we need to clone the shader,
instead of sharing the reference with gl_program, which would result
in a double free in _mesa_delete_program().

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