AVCodecContext::refs is used to control the DPB size to be used by the
encoder. The default value for AVCodecContext::refs as set in
libavcodec/options_table.h is 1.

This patch sets AVCodecContext::refs to 0 for h264_nvenc and hevc_nvenc in
order to let the driver take the decision of the correct DPB size to use in
all cases.
Signed-off-by: Srinath K R <skr@nvidia.com>
Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>