Rockchip: Remove config store usage in encoder plugin.

The plugin always encodes one buffer at a time, synchronously, so it can just
explicitly set the controls every time before encoding. It lets us remove the
usage of the downstream Config Store API.

BUG=chromium:965379
TEST=video_VideoEncodeAccelerator.vp8 unittests on device.

Change-Id: I2788a42748104f1912fbb915a9b145b217ef1b8d
Signed-off-by: Francois Buergisser <fbuergisser@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1631030


Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>