drm/exynos: remove unnecessary exynos_hdmi.h file

The exynos_hdmi.h has been used for the dedicated i2c drivers
that were already removed. Thus, the unnecessary exynos_hdmi.h
should be removed.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>