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

Need to adjust the backend map depending on which RB is
enabled.  This is the trinity equivalent of:
f7eb9730

May fix:
https://bugs.freedesktop.org/show_bug.cgi?id=57919



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org