drm/exynos: fixed overlay data updating.

this patch adds common members to overlay structure and
makes each driver such as fimd or hdmi driver set them to
its own structure.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>