omx: call handle_messages() only once in acquire_buffer() to avoid potential deadlock

There is one rare case where calling handle_messages() more than once can cause a deadlock
in the video decoder element:

- sink pad thread starts the src pad task (gst_omx_video_dec_loop())
- _video_dec_loop() calls gst_omx_port_acquire_buffer() on dec_out_port
- blocks in gst_omx_component_wait_message() releasing comp->lock and comp->messages_lock
  (initially, there are no buffers configured on that port, so it waits for OMX_EventPortSettingsChanged)
- the sink pad thread pushes a buffer to the decoder with gst_omx_port_release_buffer()
- _release_buffer() grabs comp->lock and sends the buffer to OMX, which consumes it immediately
- EmptyBufferDone gets called at this point, which signals _wait_message() to unblock
- the message from EmptyBufferDone is processed in gst_omx_component_handle_messages()
  called from gst_omx_port_release_buffer()
- gst_omx_port_release_buffer releases comp->lock
- the src pad thread now gets to run, grabbing comp->lock while it exits from _wait_message()
- _acquire_buffer() calls the _handle_messages() on the next line after _wait_message(),
  which does nothing (no pending messages)
- then it goes to "retry:" and calls _handle_messages() again, which also does nothing
  (still no pending messages)
- scheduler switches to a videocore thread that calls EventHandler, informing us about the
  OMX_EventPortSettingsChanged event that just arrived
- EventHandler graps comp->messages_lock, but not comp->lock, so it can run in parallel at
  this point just fine.
- scheduler switches back to the src pad thread (which is in the middle of _acquire_buffer())
- the next _handle_messages() which is right before if (g_queue_is_empty (&port->pending_buffers))
  processes the OMX_EventPortSettingsChanged
- the buffer queue is still empty, so that thread blocks again in _wait_message()
- the sink pad thread tries to acquire the next input port buffer
- _acquire_buffer() also blocks this thread in:
   if (comp->pending_reconfigure_outports) { ... _wait_message() ... }
- DEADLOCK. gstreamer is waiting for omx to do something, omx waits for gstreamer to do something.

By removing those extra _handle_messages() calls, we can ensure that all the checks of
_acquire_buffer() will re-run. In the above case, after the scheduler switches back to
the middle of _acquire_buffer(), the code will enter _wait_message(), which will see that
there are pending messages and will return immediately, going back to "retry:" and
re-doing all the checks properly.

https://bugzilla.gnome.org/show_bug.cgi?id=741854

omx/gstomx.c

@@ -1326,12 +1326,10 @@ retry:
    * arrives, an error happens, the port is flushing
    * or the port needs to be reconfigured.
    */
-  gst_omx_component_handle_messages (comp);
   if (g_queue_is_empty (&port->pending_buffers)) {
     GST_DEBUG_OBJECT (comp->parent, "Queue of %s port %u is empty",
         comp->name, port->index);
     gst_omx_component_wait_message (comp, GST_CLOCK_TIME_NONE);
-    gst_omx_component_handle_messages (comp);
 
     /* And now check everything again and maybe get a buffer */
     goto retry;