radeonsi: implement forcing per-sample_interpolation using the shader key only

radeonsi: implement forcing per-sample_interpolation using the shader key only

It was partly a state and partly emulated by shader code, but since we want
to do this in a fragment shader prolog, we need to put it into the shader
key, which will be used to generate the prolog.

This also removes the spi_ps_input states and moves the registers
to the PS state.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>