i965: Remove legacy clip plane handling from geometry shaders.

i965: Remove legacy clip plane handling from geometry shaders.

We only support geometry shaders in core profiles, where gl_ClipVertex
doesn't exist.  Presumably the even older behavior of clipping to
gl_Position isn't supported either.  In fact, GLSL 1.50 page 76 claims:

"The shader must also set all values in gl_ClipDistance that have been
 enabled via the OpenGL API, or results are undefined."

So we don't need to handle legacy clipping in geometry shaders.  I think
Paul added this back when we were considering supporting the old
GL_ARB_geometry_shader4 extension.

This removes a non-orthagonal state dependency on GS compilation.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>