drm/exynos: cleanup exynos_hdmi.h

The some contents of the exynos_hdmi.h are used only in exynos_hdmi.c,
so move them to exynos_hdmi.c.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>