rnndb/display: Add REPORT_UNDERFLOW register

doc: ftp://download.nvidia.com/open-gpu-doc/gk104-disable-underflow-reporting/1/gk104-disable-underflow-reporting.txt

Reported-by: Andy Ritger <aritger@nvidia.com>
Signed-off-by: Roy Spliet <r.spliet@student.tudelft.nl>