Make the spatially adaptive mode search in rate-distortion
optimization loop inter tile independent. Experiments suggest that
this does not significantly change the coding staticstics.

Single tile, speed 3:
pedestrian_area 1080p 1500 kbps
59192 b/f, 40.611 dB, 101689 ms

blue_sky 1080p 1500 kbps
58505 b/f, 36.347 dB, 62458 ms

mobile_cal 720p 1000 kbps
13335 b/f, 35.646 dB, 45655 ms

as compared to 4 column tiles, speed 3:
pedestrian_area 1080p 1500 kbps
59329 b/f, 40.597 dB, 101917 ms

blue_sky 1080p 1500 kbps
58712 b/f, 36.320 dB, 62693 ms

mobile_cal 720p 1000 kbps
13191 b/f, 35.485 dB, 45319 ms

Change-Id: I35c6e1e0a859fece8f4145dec28623cbc6a12325