vp9_ethread: modify VP9_COMP structure

This patch modified struct VP9_COMP. Created a struct ThreadData
to include data that need to be copied for each thread. In
multiple thread case, one thread processes one tile. all threads
share one copy of VP9_COMP,
(refer to VP9_COMP *cpi in the code)
but each thread has its own copy of ThreadData,
(refer to ThreadData *td in the code).
Therefore, within the scope of encode_tiles(), both cpi and td
need to be passed as function parameters.

In single thread case, the FRAME_COUNTS pointer in ThreadData
points to "counts" in VP9_COMMON.

Change-Id: Ib37908b2d8e2c0f4f9c18f38017df5ce60e8b13e