drm/tegra: Changes for v5.6-rc1

This contains a small set of mostly fixes and some minor improvements.
Signed-off-by: Dave Airlie <airlied@redhat.com>

From: Thierry Reding <thierry.reding@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200111004835.2412858-1-thierry.reding@gmail.com