drm/i915: Remove redundant parameter to i915_gem_object_wait_rendering__tail()

An earlier commit (c8725f3d: Do not call
retire_requests from wait_for_rendering) removed the use of the ring parameter
within wait_rendering__tail() but did not remove the parameter itself. As the
plan is to remove obj->ring which is where this parameter comes from, it is
simpler to just remove the parameter completely than to update it with a new
source.

For: VIZ-4377
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
CC: Chris Wilson <chris@chris-wilson.co.uk>
CC: Brad Volkin <bradley.d.volkin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>