drm/exynos/mixer: always update INT_EN cache

INT_EN cache field was updated only by mixer_enable_vblank.
The patch adds update also by mixer_disable_vblank function.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>