drm/exynos: dpi: Add support for panel prepare and unprepare routines

Modify exynos_dpi driver to support the new panel calls:
prepare and unprepare.

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>