drm/exynos: remove extra call to exynos_dp_commit()

exynos_dp_commit() was getting called twice by exynos encoder core, once
inside the .enable() call and another time by .commit() itself.

The remove of the second call caused the wake of a bug, the operations
orders inside exynos_dp_commit was wrong and we had to move
exynos_dp_start_video() to be the last operation in there.

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