drm/exynos: Remove unnecessary code

Function usage exynos_atomic_commit was removed in
commit 41cbf0f ("drm/exynos: use atomic helper commit").
Remove unnecessary function declare.

Signed-off-by: Kwanghoon Son <k.son@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

Author: KwangSon

drivers/gpu/drm/exynos/exynos_drm_drv.h

@@ -254,10 +254,6 @@ static inline int exynos_drm_check_fimc_device(struct device *dev)
 }
 #endif
 
-int exynos_atomic_commit(struct drm_device *dev, struct drm_atomic_state *state,
-			 bool nonblock);
-
-
 extern struct platform_driver fimd_driver;
 extern struct platform_driver exynos5433_decon_driver;
 extern struct platform_driver decon_driver;