drm/rockchip: use drm_for_each_connector_iter()

Drop legacy drm_for_each_connector() in favor of the race-free
drm_for_each_connector_iter()

Cc: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170515134330.3275-1-gustavo@padovan.org