drm/exynos: Add mode_set manager operation

This patch adds a mode_set callback to the manager operations which
sets the crtc's current mode to the manager driver. This will allow the
fimd  driver to set its mode using values from drm, instead of the dt.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>