drm/radeon/kms: fix bandwidth calculation when sideport is present

Fixes fdo bug 27529:
https://bugs.freedesktop.org/show_bug.cgi?id=27529



Reported-by:  <steckdenis@yahoo.fr>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>