drm/exynos: Move DP setup into commit()

Add commit callback for exynos_dp, and move the DP link training,
video configuration code from the hotplug handler into commit().

Signed-off-by: Sean Paul <seanpaul@chromium.org>
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>