This is needed for proper synchronization with display on another DRM
device (pl111 or tinydrm) with buffers produced by vc4 V3D.  Fixes the
new igt vc4_dmabuf_poll testcase, and rendering of one of the glmark2
desktop tests on pl111+vc4.

This doesn't yet introduce waits on another device's fences before
vc4's rendering/display, because I don't have testcases for them.

v2: Reuse dma_fence_free(), retitle commit message to clarify that
    it's not a full dma-buf fencing implementation yet.
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170412191202.22740-6-eric@anholt.netAcked-by: Daniel Vetter <daniel.vetter@ffwll.ch>