Subpel search cleanups and enhancements

Subpel search cleanups and enhancements

- Some fixes to surface fit.
- Returns variance function as cost rather than sad in the
  pattern search and diamond search functions. Only
  vp9_pattern_search_sad function used in bigdia search
  uses sad as integer 1-away costs.
- Deploys SUBPEL_TREE_PRUNED_MORE for speed 4+.

Results:
derf [Speed 3]: About +0.036% in coding efficiency without any
discernible speed loss.
derf [Speed 4]: About 2-3% faster at -0.199% loss in coding efficiency.
derf [Speed 5]: About 3-4% faster at -0.149% loss in coding efficiency.

Change-Id: I8462f94f6adb46966ca964f2bd0400977357fd63