drm/exynos: add plane enable/disable

The plane enable/disable can control only a power of plane, so they will
be helpful to handle planes with dpms.

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>