nouveau/video: make sure that firmware is present when checking caps

nouveau/video: make sure that firmware is present when checking caps

Apparently some players are ill-prepared for us claiming that a decoder
exists only to have creating it fail, and express this poor preparation
with crashes (e.g. flash). Check that firmware is there to increase the
chances of there being a high correlation between reported capabilities
and ability to create a decoder.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: 10.0 10.1 <mesa-stable@lists.freedesktop.org>
Tested-by: Emil Velikov <emil.l.velikov@gmail.com>