drm/nouveau: Fix bandwidth calculation for DisplayPort

Ported from the equivalent fix in drm-intel-next:

http://cgit.freedesktop.org/~keithp/linux/commit/?h=drm-intel-next&id=cd9dde44



Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>