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

This function is valid only the SoC has SHADOWCON register
and it should be used together with fimd_enable_video_output()
to match the ENWIN_F bit in WINCON# and C#_EN_F bit in SHADOWCON.

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>