vp9_ethread: include a pointer to mb in VP9_COMP

Modified VP9_COMP struct to include MACROBLOCK *mb. This change
makes it feasible in multi-thread case to allocate a mb for each
thread.

Change-Id: I624d6d1aa9c132362200753e5d90b581b1738d6e
Merge "vp9_ethread: include a pointer to mb in VP9_COMP"