drm/exynos: use prime helpers

The dma-buf codes of exynos drm is almost same with prime helpers. A
difference is that consider DMA_NONE when import dma-buf, but it's wrong
and we don't consider it any more, so we can use prime interface.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>