There was a VDPAU fix in MPV 0.6.1 that fixed this kind of lockup on my
hardware: RS880G.

My videos are all 1080i50 H.264 recordings from DVB-S2 broadcasts of various
channels. Since they are just chunks of transport stream, I think that lockups
when starting a video and when seeking would be similar.

Also I have ~oibaf MESA from 4th October, the first with the VAAPI state
tracker, and I tested that too. Here is a summary of what I found, with MPV
0.6.1

--hwdec=vdpau --vo=vdpau does not lock up the GPU but...

--hwdec=vdpau --vo=vdpau --deinterlace locks the GPU immediately.

I assume deinterlace will not work until there is a proper workaround for the
frame based output. UVD is not useful for me without deinterlace.

--hwdec=vaapi --vo=vaapi locks up the GPU quite often when starting to play a
video, similar to vdpau on MPV 0.6.0

--hwdec=vaapi --vo=opengl does not lock up the GPU but there is quite a lot of
coloured or speckled picture corruption on my video.