i965: Gen4-5: Don't enable hardware alpha test with MRT

We have to do this in the shader instead, since these gens lack an
independent RT0 alpha value in their render target write messages.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Eric Anholt <eric@anholt.net>

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

@@ -187,7 +187,8 @@ static void upload_cc_unit(struct brw_context *brw)
 				eqA != eqRGB);
    }
 
-   if (ctx->Color.AlphaEnabled) {
+   /* _NEW_BUFFERS */
+   if (ctx->Color.AlphaEnabled && ctx->DrawBuffer->_NumColorDrawBuffers <= 1) {
       cc->cc3.alpha_test = 1;
       cc->cc3.alpha_test_func =
 	 intel_translate_compare_func(ctx->Color.AlphaFunc);