vp3: integrate clear_blocks with idct of previous block.

This is identical to what e.g. vp8 does, and prevents the function call
overhead (plus dependency on dsputil for this particular function).

Arm asm updated by Janne Grunau <janne-libav@jannau.net>.

Signed-off-by: Janne Grunau <janne-libav@jannau.net>
