Set adaptive_rd_thresh to 0 at speed 0. This allows a thorough mode
search, and eliminates a blocking artifact seen in an encoder test.

Borg test:
1. lowres
Overall PSNR: -0.135%; SSIM: -0.293%;
2. hdres
Overall PSNR: -0.122%; SSIM: -0.208%;
Encoder speed tests: 2% - 6% slower.

Change-Id: Ie7601cb8824df8f6f2ae0b2942bd938600f76990