videodecoder: try to negotiate the buffer pool even though there is no o/p format

We could have allocation query before caps event and even without caps inside
the query. In such cases , the downstream can return a bufferpool object with
out actually configuring it. This feature is helpful to negotiate the bufferpool
with out knowing the output video format. For eg: some hardware accelerated
decoders can interpret the o/p video format only after it finishes the decoding
of one buffer at least.

https://bugzilla.gnome.org/show_bug.cgi?id=687183