glsl: fix gl_{Clip,Cull}Distance error messages

The error message in the linker that checked gl_MaxCombinedClipAndCullDistances would never be issued because the compiler was already doing the check. I think the compiler might have been done this way in the original commit d656736b as the linker only sets the size when the clip/cull outputs are written so the piglit test for this wouldn't have been triggered as it does not write to the outputs.

Here we move the error to the compiler and fix things up so the correct messages are triggered.

Fixes: d656736b ("glsl: Add arb_cull_distance support (v3)")