glsl: Make sure gl_ClipDistance and gl_ClipVertex are not both written.

glsl: Make sure gl_ClipDistance and gl_ClipVertex are not both written.

From section 7.1 (Vertex Shader Special Variables) of the GLSL 1.30
spec:

  "It is an error for a shader to statically write both
  gl_ClipVertex and gl_ClipDistance."

Fixes piglit test mixing-clip-distance-and-clip-vertex-disallowed.c.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>