drm/i915: Make i915_gem_retire_requests_ring() static

Its last usage outside of i915_gem.c was removed in:

  commit 1f70999f
  Author: Chris Wilson <chris@chris-wilson.co.uk>
  Date:   Mon Jan 27 22:43:07 2014 +0000

     drm/i915: Prevent recursion by retiring requests when the ring is full

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>