drm/radeon/kms: fix backend map typo on juniper

commit 9a4a0b9c upstream.

Don't enable backends that don't exist.

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>