drm/exynos: rotator: convert to common clock framework

This driver was not used after introduction of common clock framework.
This patch adds missing prepare/unprepare calls and allows to use it
again with current kernel code.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>