drm/exynos: remove extra declaration of struct exynos_overlay

The struct is defined in the same file, declare it here is just
unnecessary
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_drv.h

@@ -37,7 +37,6 @@
 #define wait_for(COND, MS) _wait_for(COND, MS)
 
 struct drm_device;
-struct exynos_drm_overlay;
 struct drm_connector;
 
 /* This enumerates device type. */