drm fb helper: use drm_helper_connector_dpms to do dpms

drm_fb_helper_on|off currently manually searches for encoders
to turn on/off. Make this simpler by using the helper function.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>