drm/exynos: fimd: add triggering unset routine in fimd_trigger()

There is a case like set config which requires triggering
but vblank is not enabled yet.
So triggering unset routine is required to exit from
triggering mode.

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>