drm/mediatek: use drm_for_each_connector_iter()

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

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511191049.28944-5-gustavo@padovan.org