drm: exynos: use common helper for a scatterlist contiguity check

Use common helper for checking the contiguity of the imported dma-buf.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by : Inki Dae <inki.dae@samsung.com>