drm/tegra: Changes for v5.3-rc1

This contains a couple of small improvements and cleanups for the Tegra
DRM driver.

Signed-off-by: Dave Airlie <airlied@redhat.com>

From: Thierry Reding <thierry.reding@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621150753.19550-1-thierry.reding@gmail.com