drm/arm/hdlcd: Use drm_mode_config_helper_suspend/resume()

Replace driver's code with the generic helpers that do the same thing
including the NULL check.

Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>