vaapipluginbase: Update the pool if there was no pool in the downstream reply
vaapipluginbase: Update the pool if there was no pool in the downstream reply

Fix regression introduced by bd866479, the query after decide_allocation()
always needs a pool in the first slot.

Signed-off-by: Víctor Manuel Jáquez Leal <victorx.jaquez@intel.com>

https://bugzilla.gnome.org/show_bug.cgi?id=748559

gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi