drm/tegra: dc: Avoid reset asserts on Tegra20

Commit 33a8eb8d ("drm/tegra: dc: Implement runtime PM") introduced
HW reset control. It causes a hang on Tegra20 if both display
controllers are utilized (RGB panel and HDMI). The TRM suggests that
each display controller has its own reset control, apparently it is not
correct.

Fixes: 33a8eb8d ("drm/tegra: dc: Implement runtime PM")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>