intel-gem: Emit an MI_FLUSH at glFlush() so frontbuffer rendering is flushed.

We have something similar in the X Server that covers X Server rendering, this
is the equivalent here for rendering to the front buffer.  If we cared about
avoiding this at glFlush time, we could only do this when some actual
frontbuffer rendering had occurred.

Bug #16392.

src/mesa/drivers/dri/intel/intel_context.c

@@ -365,6 +365,12 @@ intelFlush(GLcontext * ctx)
    if (!IS_965(intel->intelScreen->deviceID))
       INTEL_FIREVERTICES(intel);
 
+   /* Emit a flush so that any frontbuffer rendering that might have occurred
+    * lands onscreen in a timely manner, even if the X Server doesn't trigger
+    * a flush for us.
+    */
+   intel_batchbuffer_emit_mi_flush(intel->batch);
+
    if (intel->batch->map != intel->batch->ptr)
       intel_batchbuffer_flush(intel->batch);
 }