drm/i915: Remove vestigal DRI1 ring quiescing code

After the removal of DRI1, all access to the rings are through requests
and so we can always be sure that there is a request to wait upon to
free up available space. The fallback code only existed so that we could
quiesce the GPU following unmediated access by DRI1.

v2: Rebase

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