nir: Add "viewport vector" system values

nir: Add "viewport vector" system values

While a partial set of viewport system values exist, these are scalar
values, which is a poor fit for viewport transformations on vector ISAs
like Midgard (where the vec3 values for scale and offset each need to be
coherent in a vec4 uniform slot to take advantage of vectorized
transform math). This patch adds vec3 scale/offset fields corresponding
to the 3D Gallium viewport / glViewport+depth

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Eric Anholt <eric@anholt.net>