drm/tegra: dc: Restore coupling of display controllers

drm/tegra: dc: Restore coupling of display controllers

BugLink: https://bugs.launchpad.net/bugs/1923069
commit a31500fe7055451ed9043c8fff938dfa6f70ee37 upstream.

Coupling of display controllers used to rely on runtime PM to take the
companion controller out of reset. Commit fd67e9c6ed5a ("drm/tegra: Do
not implement runtime PM") accidentally broke this when runtime PM was
removed.

Restore this functionality by reusing the hierarchical host1x client
suspend/resume infrastructure that's similar to runtime PM and which
perfectly fits this use-case.

Fixes: fd67e9c6ed5a ("drm/tegra: Do not implement runtime PM")
Reported-by: Dmitry Osipenko <digetx@gmail.com>
Reported-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>