When making a new P-frame when B-frames are present the previous P-frame
is normally in the DPB because it will be referred to by subsequent
B-frames.  However, this is not true if there are no B-frames, or in edge
cases where a GOP ends with two P-frames.  Fix this by adding the direct
ref pics to the RPS explicitly.

Fixes #7699.
Tested-by: NUllysses A Eoff <ullysses.a.eoff@intel.com>