exynos/drm: hdmi: cleanup for hdmi common device registration

The hdmi common device registration function does not need extern definition
and for error case and unregister case, exynos_drm_hdmi_pdev should be cleared.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>