If a block's motion_mode is WARPED_CAUSAL and its mode is NEARESTMV, search
its immediate above and left neighbors to get the set of neighbor blocks
using WARPED_CAUSAL motion mode, pick the one with largest block size, and
use that neighbor's warped motion parameters directly for the current block.
If none of the neighbors uses WARPED_CAUSAL motion mode, we estimate the
current block's warped motion parameters.

Before this patch, for every block, we estimate its warped motion parameters.
With this patch, we reduce the number of blocks doing parameter estimation.
Here are results by testing on clips with camera motions.
                    WARPED_CAUSAL blocks   blocks reusing parameters
station2_240p(30f):     3857                    1678
netflix_arieal(30f):     692                     223

No noticable changes in coding gain. Borg test result showed a PSNR
change of +0.006% on cam_lowres set, and -0.014% on lowres set.

Change-Id: If12387ad0ca8a1996ea4c3f1bedcb269ebf78c6c