anv: Always use point size from the shader

anv: Always use point size from the shader

There is no API for setting the point size and the shader is always
required to set it. Section 24.4:

   "If the value written to PointSize is less than or equal to zero, or
    if no value was written to PointSize, results are undefined."

As such, we can just always program PointWidthSource to Vertex. This
simplifies anv_pipeline a bit and avoids trouble when we enable the
pipeline cache and don't have writes_point_size in the prog_data.