drm/exynos/hdmi: Fix unsafe list iteration

Function hdmi_mode_fixup() used bare list_for_each entry, which was
unsafe and caused memory corruption detected by kasan.
It now uses drm_for_each_connector_iter macro, which is now recommended
by the documentation and safe.

Signed-off-by: Maciej Purski <m.purski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>