Fix an incorrect rdcost init in skip_mode
Fix an incorrect rdcost init in skip_mode

This patch brings back the coding performance of ext-skip to where it
should be. The bug was caused by a code refactoring when the previous
patch for ext-skip was merged.

Change-Id: I8fbefe8cee0b4a5340a24936a4e37f98609582e3

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