Adjust cq_level in constrained quality mode

If we are already saving a lot in bits from the target (maximum)
bitrate in the constrained quality mode, allow the quantizer
to go lower than the cq level. This hopefully will solve issues
with getting too low a bitrate and consequently poor quality for
certain videos in cq mode.

Change-Id: I1c4e8b0171fcf58f95198b3add85eea5f3c8f19f