Add best_rd breakout in intra4x4 RD loop.

Encoding time of first 50 frames of bus (speed 0) @ 1500kbps goes from
1min5.4 to 1min4.0, i.e. 2.2% faster overall.

Change-Id: I8c32f2aff9a649ce7dd49d910dc5ba16b99c3bc6