drm/exynos: fimd: get signal polarities from device tree

The patch adds code to get signal polarization setting
from device tree display-timings node.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>