drm/radeon: fill in rb backend map on evergreen/ni.

This looks to have never gotten filled in, and it seems to
 trigger a bug in mesa.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reported-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>