drm/radeon/kms/evergreen: fix backend setup

drm/radeon/kms/evergreen: fix backend setup

commit b741be82cf2079f71553af595610f17a3a3a752a upstream.

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
(6271901d828b34b27607314026deaf417f9f9b75).

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

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>