drm/exynos: remove uneeded declaration of struct dma_iommu_mapping

It is not even used in this header anymore.
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_iommu.h

@@ -40,7 +40,6 @@ static inline bool is_drm_iommu_supported(struct drm_device *drm_dev)
 
 #else
 
-struct dma_iommu_mapping;
 static inline int drm_create_iommu_mapping(struct drm_device *drm_dev)
 {
 	return 0;