drm/exynos: Add display-timing node parsing using video helper function

Add support for parsing the display-timing node using video helper
function.

The DT node parsing is done only if 'dev.of_node'
exists and the NON-DT logic is still maintained under the 'else' part.

Signed-off-by: Leela Krishna Amudala <l.krishna@samsung.com>
Signed-off-by: Vikas Sajjan <vikas.sajjan@linaro.org>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>