i965: When gl_PointSize is unwritten, default to 1.0 on Gen4-5.

i965: When gl_PointSize is unwritten, default to 1.0 on Gen4-5.

Modern GL specifications say that the point size should be 1.0 when
gl_PointSize is unwritten and the last enabled stage is a geometry
or tessellation shader.  If it's a vertex shader, though, both the
GL specs and ES 3.0 spec say that it's undefined - so since Gen4-5
only support vertex shaders, there's no actual requirement to do this.

Since there is a cost associated (an extra dirty bit, which may cause
SF_STATE to be emitted more often), it may not be a good idea.

The real benefit is that it makes all generations behave identically.
And that seems somewhat nice...

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>