drm/i915: Refactor get_fence() to use the common fence writing routine

We can also take advantage of the new 'no retire' mode for seqno waiting
to avoid having to take a reference on the old fence object whilst
flushing an existing fence.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>