drm: Remove drm_mode_list_concat()

The last user was removed in

  commit 575dc34e
  Author: Dave Airlie <airlied@redhat.com>
  Date:   Mon Sep 7 18:43:26 2009 +1000

      drm/kms: remove old std mode fallback code.

      The new code adds modes in the helper, which makes more sense
      I disliked the non-driver code adding modes.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>