drm/i915: Use 0 for the unordered context

Since commit 078dec33 ("dma-buf: add dma_fence_get_stub") the 0
fence context became an impossible match as it is used for an always
signaled fence. We can simplify our timeline tracking by knowing that 0
always means no match.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819184404.24200-1-chris@chris-wilson.co.uk
Link: https://patchwork.freedesktop.org/patch/msgid/20190819175109.5241-1-chris@chris-wilson.co.uk