drm/exynos: drm_plane: Correct exynos_drm_plane_reset

Make use of helper functions in exynos_drm_plane_reset in order to set
all default values. Currently alpha isn't set during reset.

Signed-off-by: Christoph Manszewski <c.manszewski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>