When block size is below 16x16, the encoder swap from non-RD to
RD mode for key frame coding. This largely brough back the key
frame compression performance. For vidyo1 at 1000 kbps, the key
frame coding statistics are changed

9978F, 34.183 dB, 36807 us -> 9838F, 35.020 dB, 61677 us

As compared to the full RD case
7187F, 34.930 dB, 214470 us

The overall rtc set coding performance (single key frame setting)
is improved by 1.5%.

Change-Id: I78a4ecf025d7b24ec911e85be94e01da05e77878