drm/exynos: use drm_for_each_connector_iter()

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

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511191049.28944-2-gustavo@padovan.org