drm/prime: add return check for dma_buf_fd

The dma_buf_fd() can return error when it fails to prepare fd,
so the dma_buf needs to be put.

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>