Oh I forgot that MPEG1+2 is only available via shader-based decoding on your
card. So no luck with UVD for your sample video.

See also this commit:

commit 1681bd7f2b392d0b379cb0ff43a4fac33da74762
Author: Christian König <christian.koenig@amd.com>
Date:   Wed Jul 10 15:43:16 2013 +0200

    radeon/uvd: fall back to shader based decoding for MPEG2 on UVD 2.x v2

    UVD 2.x doesn't support hardware decoding of MPEG2, just use shader
    based decoding for those chipsets.

    Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=66450

    v2: fix interlacing as well

    Signed-off-by: Christian König <christian.koenig@amd.com>