bfin: Don't use the vp3 idct functions if bitexact behaviour is expected

In the non-bitexact mode, vp3 currently decodes to the same
frame crcs as before 28f9ab702 (and the output visually looks
correct).

Signed-off-by: Martin Storsjö <martin@martin.st>
bfin: Make vp3 functions static

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