drm/exynos: Use manager_op initialize in fimd

This patch implements the intitialize manager op in fimd. This will
allow us to keep track of drm_dev in context instead of using subdev,
which in turn makes it easier to remove subdev from fimd.

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