rockchip: verify fd is encoder device in plugin_init.

BUG=chrome-os-partner:33728
TEST=run VEA unittest on pinky

Change-Id: Ia91ba4a6baf3b7179982fdacd56584a4efd07ad2
Reviewed-on: https://chromium-review.googlesource.com/236535


Reviewed-by: Pawel Osciak <posciak@chromium.org>
Commit-Queue: Wu-Cheng Li <wuchengli@chromium.org>
Tested-by: Wu-Cheng Li <wuchengli@chromium.org>