i965: Enable lower_clip_distance.

i965 requires gl_ClipDistance to be formatted as an array of 2 vec4's
(as opposed to an array of 8 floats), so enable the lowering pass that
performs this conversion.

Reviewed-by: Eric Anholt <eric@anholt.net>

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

@@ -120,6 +120,7 @@ GLboolean brwCreateContext( int api,
 	 (i == MESA_SHADER_FRAGMENT);
       ctx->ShaderCompilerOptions[i].EmitNoIndirectTemp =
 	 (i == MESA_SHADER_FRAGMENT);
+      ctx->ShaderCompilerOptions[i].LowerClipDistance = GL_TRUE;
    }
 
    ctx->Const.VertexProgram.MaxNativeInstructions = (16 * 1024);