amdgpu: screen becomes "wrong palette"-like corrupted

[mrgtwentythree]

on an Radeon RX 550 after some time the screen usually ends
up corrupted where the corruption looks wrong palette issues, ie
you can see the right info, but all the colours are wrong.

it's easy to trigger by using mpv in full screen mode. this will
instantly trigger the bug for me in at least a dozen cases. i've also
seen it twice when glxgears (probably multiple concurrent copies)
was running, but that took several hours to trigger.

however, it's not a pure palette issue. carefully watching the
display will show information from windows that should be hidden
by the movie window. eg, if i have glxgears running and then start
mpv such that the movie is fully on top of the glxgears, it is fine,
but as soon as i press 'f' the display is corrupted and i can see some
portion of the glxgears activity where that window would be if it was
actually supposed to be visible.

[mrgtwentythree]

this no longer occurs in netbsd-current, as of today. it was a problem
earlier this year still, so something reasonably recently has fixed it.

i was able to have multiple glxgears all running with mpv starting and
stopping and also entering and exiting full-screen mode, without any
issue at all.

actually, some of the (12) glxgears hung for a short time (no more than
about 3 seconds), but all recovered OK, and a couple of times it was all
less than 1s of hangs.