drm/prime: fix up handle_to_fd ioctl return value

In

commit da34242e
Author: YoungJun Cho <yj44.cho@samsung.com>
Date:   Wed Jun 26 10:21:42 2013 +0900

    drm/prime: add return check for dma_buf_fd

the failure case handling was fixed up. But in the case when we
already had the buffer exported it changed the return value:
Previously we've return 0 on success, now we return the fd.

This ABI change has been caught by i-g-t/prime_self_import/with_one_bo.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66436


Cc: YoungJun Cho <yj44.cho@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Tested-by: lu hua <huax.lu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>