mesa/st: fix st_point_size_per_vertex helper for desktop GL

This fixes upcoming point size CTS tests that do following:

1. set point size in tess ctrl shader but ignore it in eval shader
2. set point size in vert shader but set different value in eval shader
3. set point size in vert and tess eval but ignore in geometry shader

(dEQP-GL45.functional.tessellation_geometry_interaction.*)

No regression observed in ES point size tests or Piglit tests, removed branch is likely redudant to the checks below that handle the vertex program case.

Signed-off-by: Tapani Pälli tapani.palli@intel.com