drm/i915: Remove the impedance mismatch around intel_engine_enable_signaling

There is some redundancy between dma_fence->ops->enable_signaling (via
i915_fence_enable_signaling) and our backend,
intel_engine_enable_signaling() in that both levels recheck the fence
status multiple times. If we convert intel_engine_enable_signaling() to
return the information desired by dma_fence->ops->enable_signaling, we
can reduce i915_fence_enable_signaling to a simple stub and avoid
trying to reinterpret the same information.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180308140732.25090-1-chris@chris-wilson.co.uk