r600g: use a more clever way to disable per-vertex point size

r600g: use a more clever way to disable per-vertex point size

This uses point size clamping to force point size to a particular value,
making the vertex shader output irrelevant.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>