i965: Don't do legacy userclipping in non-compatibility contexts.

According to the GLSL 1.50 specification, page 76:
"The shader must also set all values in gl_ClipDistance that have been
 enabled via the OpenGL API, or results are undefined."

With this patch, we only enable clip distance writes when the shader
actually writes them.  We no longer force a value to be written when
clip planes are enabled in the API.  This could mean the first varying
slot would be used as clip distances - I believe it should be the safe
kind of undefined behavior.

Empirically, it doesn't seem to cause a problem.

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

src/mesa/drivers/dri/i965/brw_vs.c

@@ -308,6 +308,7 @@ brw_vs_populate_key(struct brw_context *brw,
    key->program_string_id = vp->id;
 
    if (ctx->Transform.ClipPlanesEnabled != 0 &&
+       ctx->API == API_OPENGL_COMPAT &&
        !vp->program.Base.UsesClipDistanceOut) {
       key->nr_userclip_plane_consts =
          _mesa_logbase2(ctx->Transform.ClipPlanesEnabled) + 1;