vc1parser: fix level values for simple/main profile
In simple profile, level set to 0 or 2 indicate low and medium level respectively. In main profile, level set to 0, 2 or 4 indicate low, medium and high level respectively. Level values are defined in Annex J.1.2 of the SMPTE 421M. https://bugzilla.gnome.org/show_bug.cgi?id=738230