drm/exynos/hdmi: remove old platform data code

s5p_hdmi_platform_data were used before device tree introduction.
As HDMI driver is DT only we can drop this struct completely.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>