pan/va: Fix nir_op_pack_uvec4_to_uint
What does this MR do and why?
We don't have a generic v4i8 on Valhall, we have to lower it to two v2i8. Fortunately, bi_make_vec_to() hides the Bifrost/Valhall differences, so use that for nir_op_pack_uvec4_to_uint.