nir: fix bit of cargo-culting in lower_idiv

nir: fix bit of cargo-culting in lower_idiv

I guess I was looking too much at how lower_system_values worked when
writing lower_idiv.

Since ttn wasn't emitting load_var for sysvals and the only drivers
using lower_idiv were using ttn, I think nothing was broken as a result.
But might as well fix this before it becomes a problem.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>