Avoid calling get_dist_cost_stats when qc == 0
Avoid calling get_dist_cost_stats when qc == 0

This speed up encoder by 5%

Change-Id: I131f32f7c698c30f10b3f8ae7888189039f1404e

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom