avcodec/mpeg12dec: Add FF_CODEC_CAP_SKIP_FRAME_FILL_PARAM

This decreases the amount of computations and memory needed for analysing mpeg1/2 streams

the properties update is moved from code that is skiped if skip_frame is set
to code that is not skiped so the change doesnt loose that
from being executed

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>