drm/exynos: fimd: add fimd_enable_video_output() to cleanup

This bit is used for video output and logic signal control.
So it is better for readability.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>