rkvdec: Use vb2_find_buffer

Use the newly introduced vb2_find_buffer API to get a vb2_buffer
given a buffer timestamp.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>