drm: Export routines for inserting preallocated nodes into the mm manager

Required by i915 in order to avoid the allocation in the middle of
manipulating the drm_mm lists.

Use a pair of stubs to preserve the existing EXPORT_SYMBOLs for
backporting; to be removed later.

Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[danvet: bikeshedded-away the atomic parameter, it's not yet used
anywhere.]
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>