Properly reset the skip_mode element in mb_mode_info
Properly reset the skip_mode element in mb_mode_info

The skip_mode element might re-use prior frame's coding decision
for a current coding block rate-distortion search. Properly reset
it to be zero for regular rate-distortion mode search.

This improves the coding performance for ext-skip by 0.07% for
lowres.

Change-Id: Idbda5b441e3eb844e03ca07bd174b4b7f8a7cb59

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