drm/exynos: check for null in return value of dma_buf_map_attachment()

dma_buf_map_attachment() can return NULL and valid sg as return
value. Hence the check for the returned scatter-gather must be using
the inline function IS_ERR_OR_NULL() in place of IS_ERR()

Signed-off-by: Subash Patel <subash.ramaswamy@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>