drm/exynos: Replace repeated declarations by #include "exynos_drm_drv.h"

Re-declare struct is not a good practice, let's use the original
drm and exynos declarations.
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_crtc.h

@@ -15,10 +15,7 @@
 #ifndef _EXYNOS_DRM_CRTC_H_
 #define _EXYNOS_DRM_CRTC_H_
 
-struct drm_device;
-struct drm_crtc;
-struct exynos_drm_manager;
-struct exynos_drm_overlay;
+#include "exynos_drm_drv.h"
 
 int exynos_drm_crtc_create(struct exynos_drm_manager *manager);
 int exynos_drm_crtc_enable_vblank(struct drm_device *dev, int pipe);