freedreno/a6xx: Track location of gl_Position out as we link it

Kristian H. Kristensen requested to merge krh/mesa:pos-loc into master

When using xfb and rasterizing, the fragment shader may have fewer inputs than the vertex shader outputs. We can't rely on gl_Position to be places at fs->total_in, but have to instead remember where we add it in the link map and use that location.

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