rdopt,rd_pick_intra4x4block: port tsan fix from libvpx

minus the non-existent nonrd portion. original change:

commit d642294b
Author: Jingning Han <jingning@google.com>
Date:   Thu Feb 11 12:36:49 2016 -0800

    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

Change-Id: If0c11092450ede0450fc9bcf5db2f7349ac2860c