drm/exynos: dp: Modify driver to support drm_panel

Add drm_panel controls to support powerup/down of the
eDP panel, if one is present at the sink side.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>