intel: avoid unnecessary front buffer flushing/updating

Before, if we just called glXMakeCurrent() and didn't render anything we'd
still trigger a flushFrontBuffer() call.

Now only set the intel->front_buffer_dirty field at state validation time
just before we draw something.

NOTE: additional calls to intel_check_front_buffer_rendering() might be
needed if I missed some rendering paths.

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

@@ -501,6 +501,8 @@ intel_map_unmap_framebuffer(struct intel_context *intel,
       else
          intel_renderbuffer_unmap(intel, fb->_StencilBuffer->Wrapped);
    }
+
+   intel_check_front_buffer_rendering(intel);
 }
 
 /**