mpeg2dec: do not reset the whole video info when setting size

New changes to gstvideo will reset all the video info state
when calling _set_format, overwriting what was previously set
in the preceding code.

The comment says the following code is meant to preserve the
pre-crop size, so let's just keep the size and related data
 as this does not seem to break anything else (this is what
the _set_format call would have set before the change that
reset all data, except the colorimetry).