radeonsi: mpv --vo=vaapi incorrect rendering on gfx9+

Playing any video clip on gfx9+ (tested on Raven Ridge and Navi) using: mpv --vo=vaapi myvideo.mkv looks like this:

This regression has been introduced by commit 86e60bc2 (associated Merge Request: !2110 (closed))

Author: Marek Olšák <maraeo@gmail.com>
Date:   Wed Sep 18 21:17:11 2019 -0400

    radeonsi: remove si_vid_join_surfaces and use combined planar allocations
    
    Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

cc @mareko

Edited Dec 06, 2019 by Pierre-Eric Pelloux-Prayer

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