drm/prime: fix to check return of dma_map_sg in prime helper

The dma_map_sg(), in map_dma_buf callback operation of prime helper,
can return 0 when it fails to map, so it needs to release related
resources.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>