freedreno/a5xx: Don't set the VARYING flag for fragcoord-only programs.

Emma Anholt requested to merge anholt/mesa:fd-a5xx-fragcoord-only into master

As with a6xx (commits beb02a78, 5785bcc8), the blob doesn't set this flag for a5xx when fragcoords are used but not proper varyings. See for example dEQP-GLES2.functional.shaders.builtin_variable.fragcoord_xyz.

The hope was that this would clear up separate_shader fails/flakes like it helped with a6xx's flakes, but that didn't happen.

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