avcodec/nvdec: Make vp8 initialisation more 'compatible'

Ancient versions of gcc (pre 4.6) can't directly initialise
members of anonymous inner unions/structs by name.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10676

Unfortunately, RHEL 6 shipped with one of these ancient
versions and so we're stuck with it until approximately the
heat death of the universe.

Putting explicit braces into the initialisation is possibly a
work-around but the behaviour there was never fully understood
before direct initialisation was made to work.

So, this may or may not work.
