drm/i915: remove i915_gem_object_ggtt_unbind

Only has one user and is nothing more than a shim on top of
i915_vma_unbind, so let's just get rid of it.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461842691-27575-1-git-send-email-matthew.auld@intel.com