Fix tsan error in VP9 sub8x8 intra mode search

This commit fixes issue 1141. The issue was triggered in multi-tile
encoding. The change properly saves and restores the block context
information in the real-time mode selection process. It removes
several redundant memcpy operations in sub8x8 intra block mode search.

Change-Id: I35c9ad197f4bd500ec39b5fc833f052f19eee010