drm/radeon/kms/evergreen: fix backend setup

This patch fixes rendering errors on some evergreen boards.
Hardcoding the backend map is not an optimal solution, but
a better fix is being worked on.

Similar to the fix for rv740
(6271901d).

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=29986



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>