vp3: Use full transpose for all IDCTs

This way, the special IDCT permutations are no longer needed. This
is similar to how H264 does it, and removes the dsputil dependency
imposed by the scantable code.

Also remove the unused type == 0 cases from the plain C version
of the idct.

Signed-off-by: Martin Storsjö <martin@martin.st>