drm: Preserve the list head in drm_mode_copy

Preserve the destination mode's list head in drm_mode_copy. Just
in case someone decides that it's a good idea to overwrite a mode which
happens to be on some list,

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>