drm/exynos: drm_rotator: Fix incorrect usage of IS_ERR_OR_NULL

Use IS_ERR instead of IS_ERR_OR_NULL on clk_get results.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>