drm/ingenic: Add basic PM support

Call drm_mode_config_helper_suspend() and
drm_mode_config_helper_resume() on suspend and resume, respectively.

This makes sure that the display stack is properly disabled when the
hardware is put to sleep.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128171606.132830-1-paul@crapouillou.net