drm/exynos: dp: remove suspend/resume functions

This patch removes unnecessary pm suspend/resume functions.

All kms sub drivers will be controlled by top of Exynos drm driver
and connector dpms so these sub drivers shouldn't have their own
pm interfaces.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>