drm/i915: Check against the signaled bit for fences/requests

When dma_fence_signal() is called, it sets a flag to indicate the fence
is complete. Before the dma_fence is signaled, the seqno check will
first be passed. During an unlocked check (such as inside a waiter), it
is possible for the fence to be signaled even though the seqno has been
reset (by engine wraparound). In this case the waiter will be kicked,
but for an extra layer of protection we can check the persistent
signaled bit from the fence.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223074422.4125-2-chris@chris-wilson.co.uk