gallium/ttn: Convert inputs and outputs to derefs of variables.

gallium/ttn: Convert inputs and outputs to derefs of variables.

This means that TTN shaders more closely resemble GTN shaders: they have
inputs and outputs as variable derefs, with the variables having their
.driver_location already set up for you.

This will be useful for v3d to do input variable DCE in NIR, which we
can't do when the TTN shaders never have a pre-nir_lower_io stage.

Acked-by: Rob Clark <robdclark@gmail.com>