vaapidecode: set format before decide allocation
vaapidecode: set format before decide allocation

There is a regression from commit 3d8e5e. It was expected the buffer pool
allocation occur before the caps negotiation, but it is not.

This patch fixes this regression: the caps negotiation is done regardless the
allocation query from downstream.

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

https://bugzilla.gnome.org/show_bug.cgi?id=756686

gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi