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>