convert drm_atomic_helper_suspend/resume() to use
drm_mode_config_helper_suspend/resume().

With this conversion, tegra_drm_fb_suspend() and
tegra_drm_fb_resume() will not be used anymore.
Both of these functions can be removed.

Also, in tegra_drm struct's member state will not be
used anymore. So this can be removed forever.

Fixed one sparse warning.
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Ajit Negi <ajitn.linux@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>