Prevent decoder from using uninitialized entropy context.

If decoding starts with intra-only frame, there is a possibility
of using uninitialized entropy context, what leads to undefined
behavior.

Change-Id: Icbb64b5b1bd1e5de2a4bfa2884e56bc0a20840af
Use right shift to replace division in vp9_pick_inter_mode

Make the variable reduction_fac log2 based and explicitly use
right shift when computing intra_cost_penalty.

Change-Id: I208f1fb879a02debb3b3fc64f9fd06260dcf1c86
Merge "Use right shift to replace division in vp9_pick_inter_mode"
Merge "Prevent decoder from using uninitialized entropy context."