avcodec/vc1dec: Fixing is_intra, 2nd attempt

This also un-does the fate changes from a52f443714b5c2a40ed272d8445f4c39220a4b69,
leaving this fix without even small differences in the output, that is
a sample for which this makes a vissible difference is very welcome

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>