drm/exynos: restore core HDMI settings

In DVI mode the video preamble and Guard band should
be disabled whereas it should be applied in HDMI mode,
the re-applying of preamble and guard band was missing,
which resulted in display failures when switched to HDMI
mode from DVI mode.
This patch ensures the setting is applied in HDMI mode.

Signed-off-by: Shirish S <s.shirish@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>