drm/i915: Refactor fence clearing to use the common fence writing routine

Now that we have a routine that is able to clear the fences as well as
setup up the register for a tiled object, remove the surplus routines to
clear the fences.

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