ffmpeg_vaapi: fix choice of decoder_format

The check could previously never evaluate to true, probably due to
a typo.

Reported-By: Mihai Chindea <mihai.chindea@uti.eu.com>
Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Tested-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
