drm/radeon/kms: release AGP bridge at suspend

I think it's good to release the AGP bridge at suspend
and reacquire it at resume. Also fix :
https://bugzilla.kernel.org/show_bug.cgi?id=15969



Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>