drm/exynos: fix to pointer manager member of struct exynos_drm_subdrv

drm/exynos: fix to pointer manager member of struct exynos_drm_subdrv

The struct exynos_drm_manager has to exist for exynos drm sub driver
using encoder and connector. If it isn't NULL to member of struct
exynos_drm_subdrv, will create encoder and connector else will not. And
the is_local member also doesn't need.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>