Add get_drl_cost()
Add get_drl_cost()

This CL will facilitate moving the inter related cost into
handle_inter_mode.

Change-Id: I3aaf22d873e1fcb533f4ec60f5a829040917817d

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