h264decoder: Fix DPB bumping process
As per spec C.4.5.3 "Bumping", if bumping is needed but DPB holds no "output needed" picture, then a picture that has the smallest POC should be considered first for output
As per spec C.4.5.3 "Bumping", if bumping is needed but DPB holds no "output needed" picture, then a picture that has the smallest POC should be considered first for output