drm/exynos: remove exynos_drm_rotator.h

Since its inclusion in 2012 via commit bea8a429

 ("drm/exynos: add rotator ipp driver")
this header is not used by any source files and is empty.
Lets just remove it.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>