Improve enhanced_full_pixel_motion_search

Improve enhanced_full_pixel_motion_search

Do full pixel MV search around all 3 MV candidates.

Coding gains for speed 0:
         avg_psnr   ovr_psnr   ssim
lowres   -0.088%    -0.095%   -0.117%
midres   -0.175%    -0.177%   -0.148%
hdres    -0.115%    -0.146%   -0.146%

Coding gains for speed 1:
         avg_psnr   ovr_psnr   ssim
lowres   -0.089%    -0.104%   -0.124%
midres   -0.151%    -0.171%   -0.195%
hdres    -0.110%    -0.105%   -0.132%

Tested encoding speed with speed 1 QP=30,40 over 10 midres sequences,
average speed loss is about 1%.

Change-Id: I9e6de035f4ed2e814e6494aefc2f84aae333a6b4