glsl: simplify ES Vertex/Fragment shader requirements

glsl: simplify ES Vertex/Fragment shader requirements

We really just needed to skip the existing ES < 3.1 check if we have
a compute shader, all other scenarios are already covered.

* No shaders is a link error.
* Geom or Tess without Vertex is a link error which means we always
  require a Vertex shader and hence a Fragment shader.
* Finally a Compute shader linked with any other stage is a link error.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>