drm/i915: Remove i915_gem_object_check_coherency()

This code was dead since:

  commit 432e58ed
  Author: Chris Wilson <chris@chris-wilson.co.uk>
  Date:   Thu Nov 25 19:32:06 2010 +0000

      drm/i915: Avoid allocation for execbuffer object list

so just put it to rest for good.

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