st/mesa: keep serialized NIR instead of nir_shader in st_program

st/mesa: keep serialized NIR instead of nir_shader in st_program

This decreases memory usage, because serialized NIR is more compact.

If shader_has_one_variant is true and the shader is uncached, the first
variant is created from nir_shader, otherwise the first variant and
all other variants are created from serialized NIR.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>