h264: support process for gaps in frame_num

Follow 8.2.5.2 when frame_num is not equeal to
PreRefFrameNum and is not equal to
(PreRefFrameNum+1)%MaxFrameNum, there would be
"non-existing" picture "used for short-term reference".

Because the driver needs to test the effectiveness
of the picture, dummy picture needs a surfaceID.
And according to the provisions of spec, the dummy
pictures is not used for reference,so surfaceID of
current picture is set to the dummy picture.

Signed-off-by: Zhong Cong <congx.zhong@intel.com>
initPictureRefsPicNum never called in decoder before, can we move entire while block  to a member function of   DPBManager?