drm/vc4: Release fence after signalling

We were never releasing the initial fence reference that is obtained
through dma_fence_init.

Link: https://github.com/anholt/linux/issues/122
Fixes: cdec4d36

 ("drm/vc4: Expose dma-buf fences for V3D rendering.")
Signed-off-by: Stefan Schake <stschake@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1512236444-301-1-git-send-email-stschake@gmail.com