A couple of minor fixes to the display changes that went in for 5.9.
The most important of which is a workaround for a HW bug that was
exposed by better push buffer space management, leading to
random(ish...) display engine hangs.

Signed-off-by: Dave Airlie <airlied@redhat.com>
From: Ben Skeggs <skeggsb@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ <CACAvsv5QDxyMihrxbPk+-sORnaYtjR6_dbM68gEhb2wxht_G1w@mail.gmail.com