(In reply to mikhail.v.gavrilov from comment #11)
> (In reply to Christian König from comment #10)
> > Please provide the output of vainfo.
> 
> $ vainfo
> libva info: VA-API version 1.1.0
...

At least of hand that looks like it should work.

Not the slightest idea what's going wrong here, but it must be something
specific to your system.

Maybe try different combination of mpv parameters. E.g. explicitly specify
which hw decoding and which output driver to use.