commit c8435362 upstream.

A few reports of bad behaviour since the autodetection defaulted to 6bpc,
lets fix this.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>