drm/exynos: Remove exynos_drm_hdmi shim

This patch trims exynos_drm_hdmi out of the driver. The reason it
existed in the first place was to make up for the mixture of
display/overlay/manager ops being spread across hdmi and mixer. With
that code now rationalized, mixer and hdmi map directly to
exynos_drm_crtc and exynos_drm_encoder, respectively. Since there is a
1:1 mapping, we no longer need this layer.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>