vGEM buffers are useful for passing data between software clients and
hardware renders. By allowing the user to create and attach fences to
the exported vGEM buffers (on the dma-buf), the user can implement a
deferred renderer and queue hardware operations like flipping and then
signal the buffer readiness (i.e. this allows the user to schedule
operations out-of-order, but have them complete in-order).

This also makes it much easier to write tightly controlled testcases for
dma-buf fencing and signaling between hardware drivers.

v2: Don't pretend the fences exist in an ordered timeline, but allocate
a separate fence-context for each fence so that the fences are
unordered.
v3: Make the debug output more interesting, and show the signaled status.
v4: Automatically signal the fence to prevent userspace from
indefinitely hanging drivers.

Testcase: igt/vgem_basic/dmabuf-fence
Testcase: igt/vgem_slow/nohang
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Zach Reizner <zachr@google.com>
Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Zach Reizner <zachr@google.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468571471-12610-1-git-send-email-chris@chris-wilson.co.uk