nir/lower_fragcolor: handle dual source blending
With dual source blending only 1 draw buffer could be used.
For zink fixes tests:
arb_blend_func_extended-fbo-extended-blend-pattern_gles2
I'm not sure at which exact point things go wrong without this fix, but I think it's definitely wrong to write into draw buffers that are above the limit.
Found while fixing the same test for freedreno.