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

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

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 placed at fs->total_in, but have to instead remember where we add
it in the link map and use that location.

Fixes 100+ tesselation dEQPs under

  dEQP-GLES31.functional.tessellation.primitive_discard.*
  dEQP-GLES31.functional.tessellation.user_defined_io.*

Reviewed-by: Eric Anholt <eric@anholt.net>