dma-buf: return index of the first signaled fence (v2)

Return the index of the first signaled fence.  This information
is useful in some APIs like Vulkan.

v2: rebase on drm-next (fence -> dma_fence)

Signed-off-by: monk.liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
 [sumits: fix warnings]
Link: http://patchwork.freedesktop.org/patch/msgid/1478290570-30982-1-git-send-email-alexander.deucher@amd.com