media: cedrus: Fix NULL buf dereference

This is a regression introduced after porting to vb2_find_buffer.
The function returnis NULL when the buffer isn't found. The HEVC
decoder would call a helper to get the motion vector buffer address
by passing the index. This is fixed by passing the buffer
pointer instead of the index.

Fixes: 7f361451 ("cedrus: Use vb2_find_buffer")
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>