nir: Define system values for vc4's blending-lowering arguments.

nir: Define system values for vc4's blending-lowering arguments.

In the GLSL-to-NIR conversion of VC4, I had a bit of trouble with what I
was calling the "state uniforms" that I was putting into the NIR fighting
with its other lowering passes.  Instead of using magic uniform base
numbers in the backend, follow the lead of load_user_clip_plane and just
define system values for them.

v2: Fix unintended change to channel_num, drop unspecified const_index
    value on blend_const_color_r_float.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>