libvpx

Increase disable_filter_search_var_thresh threshold

Increased threshold(t) for interp filter search. This sped up the
encoder with some PSNR loss.

Borg tests were ran at speed 2.
t = 100, PSNR loss:
-0.710%(derf); -0.561%(stdhd); -0.647%(youtube)
speedup:
9%(derf); 3%(stdhd); 5.7%(youtube)

t = 500, PSNR loss:
-1.687%(derf); -1.665%(stdhd); -1.664%(youtube)
speedup:
18%(derf); 10%(stdhd); 8%(youtube)

Change-Id: I180e3657c1e156aaa88dc7c437f8bcbd19f5caba