drm/radeon: fix backend map setup on 1 RB sumo boards

Need to adjust the backend map depending on which
RB is enabled.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=892233



Reported-by: Mikko Tiihonen <mikko.tiihonen@iki.fi>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org